RefinishingRefinishing Tutorial: Firearm Metal Finishing

Refinishing Tutorial: Firearm Metal Finishing

(updated 04-09-2020)
I have developed my technique over 25 years, building on a childhood of extensive fine-scale model building. While there are no real secrets, it is as much art as science and nothing beats practice. I recommend magazines for practice, because they are relatively inexpensive and have features (large flats, sharp and rolled edges) that make it easy to assess your progress, and their function is not diminished by cosmetic blemishes.

My basic technique is thorough surface preparation, phosphate coating, and polymer (paint) final finish.
The gun finishing process commonly known as “paint over park” can be a fun and exciting process for both the home hobby gunsmith and the retail gun shop. This tutorial will give you a better understanding of the process, and help you to decide if it is a venture you wish to undertake yourself. Arizona Response Systems offers high quality”paint over park” for firearms (METACOL III Polymer coating over Manganese Phosphate) . Check the website for options and prices.

“Parkerizing” is a trade name owned by the Parker family, which developed the process in the late 1800s. It is currently owned by Parker-AmChem Company, but has become so common in today’s language that it is frequently used as a generic term rather than a brand name. The process covers a wide variety of phosphate deposition coatings. The most common for the military firearms industry are zinc phosphate and manganese phosphate.

The phosphate deposition process leaves a non-reflective coating on ferrous metal. This coating is an adequate protection against corrosion and abrasion in its own right, but additionally acts as a sponge to absorb oil for greater corrosion resistance and is an excellent primer coat for paint. The process can be adjusted to result in a variety of grain structures, from coarse to superfine, and in a variety of coating thickness usually measured in mass/surface area. Most commonly, it is an immersion process, but it can also be applied in a spray. Phosphates have excellent anti-galling properties as well.

You cannot phosphate stainless steel, aluminum, brass, or copper. Best results are on carbon steel. Steels with high nickel content such as some older Winchester parts, and AK-47 bolt-carriers may not take as well. Zinc phosphate is a light to medium gray, depending on hardness of base metal. Manganese phosphate runs a medium to charcoal gray. Manganese phosphate is most popular with my customers.

An expanded, video version of this tutorial is included in my “Gunplumber’s Guide to Gunsmithing: Kalashnikov Rifles,” available in the Web Store.

Israeli FAL with Manganese Phosphate (Parkerizing)
Romanian AKM in “Paint over Park”
Sand Blasting
For this tutorial, I’ll use the AK rifle as an example. Surface prep is EVERYTHING. It is the most important step and the one most neglected by amateurs.

I run my parts through an ultrasonic cleaning tank to remove any oils before sand blasting. You cannot sandblast grease – it just pushes around. Thorough cleaning in hot soapy water will make the blasting step much easier. Blow dry with compressed air.

You may get some spot rust during this process, but it will be more of a dust layer and will come right off in the sand blaster. If there is any doubt as to your ability to get the parts thoroughly dry, wait on the degreasing until you can progress immediately to the blasting step.

Here are barreled actions with matched bolts and carriers ready for sand-blasting.
Sand blasting your first few rifles can be fun. After you’ve done a couple thousand rifles, it isn’t fun anymore. I have developed what I believe to be the most time effective technique that still yields satisfactory results. After plugging or masking any surfaces you do not want blasted (hard chrome), start at the muzzle and blast all forward facing surfaces. If your blaster is small, you pull the barrel into one of the glove holes. Pay particular attention to the trigger guard, front sight post, and front trunion at the barrel pin.
Flip it over and blast all the rearward facing surfaces. Pay particular attention to the rear sight base, trigger guard, inside of rear trunion, inside of front sight.
Dividing the barreled action into convenient sections, blast while rotating. Pay p[articular attention to the rivets and the receiver. You want to get all rivet surfaces and the receiver to an equal texture. The inside of the receiver is not as critical from a cosmetic standpoint. Just make sure all the paint is removed from the trunions. Work your way up the receiver in sections.
Finished receiver. Remove barrel and chamber plug.
If you are going to paint over park, the cosmetics of the phosphated surface is not critical. If you are going to have phosphate as your final product, the cosmetics become much more important. After blasting, some areas will leach oil – anywhere that two pieces of metal join is a possible spot. Here the rivets from the scope rail are leaching oil.
Completely degreased and sand-blasted, ready for Parkerizing.
I’ve gotten a phosphated barelled action down to 10 minutes in the blaster. A regular painted one to 15 minutes. Occasionally I find one with a really tough, soft paint surface and those take me 25-30 minutes.

The sandblaster generates static electricity which electrically charges the fine paint and sand dust and causes it to stick to the surface of the metal. This will result in the phosphate “wiping off” as it is the dust and the not the underlying metal that takes on the color.

I give all my barreled actions a rinse in denatured alcohol immediately before going into the park tank. They can still have alcohol on them – it will just boil away. If you are using a gas or external electric burner for your tank, be careful not to ignite any alcohol drips. I got careless – once. Never again! Immersion heaters are the most expensive and complicated to set up, but the safest and most efficient to use.

Water does not cut the fine dust very well. MEK and lacquer thinner work, but are harsher and more expensive chemicals. I just recycle the alcohol – the dust settles out overnight, and I can pour almost all the alcohol into a new rinse bucket the next day. A gallon did 30 rifles with about 3/4 gallon recycled for next time.

Reasons for Using Phosphate
Phosphate coatings are desirable to the home gunsmith because they are relatively easy to do, compared to hot bluing or baked on coatings, and relatively benign. They require lower temperatures (less than 200 F.) then bluing, do not “spit” like caustic salt bluing, and the relatively benign acidic sludge is easy to collect for safe and proper disposal. Another benefit of phosphate coating is that the low operating temperature allows parts permanently bonded to plastics to be coated without melting, as they might in baked-on coatings and hot bluing. HK cocking handles and retractable stocks, and StG-58 cocking handles are examples.

One application of phosphate coatings that has been particularly useful in my business is to test questionable parts. There are some receivers and parts that are advertised as originals but are actually welded together from demiled components. Some weld jobs are beautifully surfaced and nearly impossible to detect visually, but if the item is phosphated, there will usually be a shift in color on the welded spot. Softer metals show up darker than harder metals and it is rare that a welded area will have the same color as the rest of the part. Silver soldered parts and brazing will also show up, where the braze line may have been covered by a different coating.

CZ 52, manganese phosphate.
1911 Pistol, manganese phosphate, before and after.
Process Description
In its most basic form, an item to be phosphated is physically cleaned and degreased, sand blasted to bare metal, cleaned again, rinsed, dipped into a grain refiner (but NOT rinsed) and submerged in a heated tank of phosphoric acid and zinc (or manganese, etc). The acid starts a pickling process on the surface of the metal, releasing gas and making a gas sheath of tiny bubbles around the metal. The dissolved metal come out of solution and adhere to the metal. Because there is some microscopic etching of the metal, the process is sometimes referred to as a “conversion process.” The acid pickling the metal does microscopically dissolve the metal and use the released hydrogen to make the phosphate coating integral with the surface of the metal. I call phosphating a “deposition” process, because the coating thickness can be measured as greater than the original part. It does differ slightly from a deposition process such as painting or electroplating where another substance is laid over the surface.

Those grains that do not adhere, called “flock” fall to the bottom of the tank and create an unusable “flocculent sludge.” The sludge must be periodically filtered out of the tank. When the part stops bubbling (gassing) it is removed from the tank, rinsed, inspected, then immersed in a water-displacing oil and hung to dry or wiped down. If the part is to be painted, it is dried with compressed air and not oiled.

The type of oil causes a change in color of the phosphate finish, darkening it (called “curing”).
Color changes can also be induced through long-term storage. Cosmolene (not a petroleum product, but a lanolin-based organic grease) and other gun-care products, as well as surface oxidation and years of cleaning and rubbing, cause phosphated surfaces to take on a greenish brown tint. This color, which I will call “patina” is one many gunsmiths wish to duplicate in restoring rifles, such as the M1 Garand.

Effect on Rifling
Because there is etching, however slight, custom, match grade barrels should probably be plugged. Use a rubber stopper in both ends, and if you have trouble keeping it sealed, use a brass threaded rod inside the barrel (steel might mar) and then slide your rubber stopper over both end sand tighten with a small nut and washer. Chrome-lining does not take phosphate coatings and the crud that appears on the inside of chrome lined barrels can be cleaned out with a brush during the rinse step.

Basic Equipment
  • cleaning solvents
  • cleaning container
  • air compressor
  • sand blaster
  • barrel and chamber plug (bullet and empty casing)
  • small parts basket
  • rinse tank (any material)
  • pretreatment tank (optional – any material)
  • pretreatment chemical (optional)
  • processing tank (stainless steel preferred)
  • phosphate solution
  • heating unit
  • thermometer
  • safety equipment (gloves, apron, goggles, baking soda, etc)
  • oil tank
  • water displacing oil
  • drip area (towels, hangers, etc)
Process Details
Surface preparation is best obtained through degreasing the part in a solvent such as lacquer thinner, acetone, or a heavy scrubbing in hot water with detergent. Stamped parts that have been brazed together will often leach oil from between the joints, so the process may need to be repeated several times. Avoid solvents that leave a petroleum-based film on the parts.
Sandblast the part with an 80#-120# aluminum oxide or garnet. Do not use silicon sand as the dust causes silicosis, a severe and untreatable health issue.

Blow dust off with compressed air, and immediately before putting in the phosphate tank, rinse with denatured alcohol. MEK or Acetone will also work, but are more expensive and more hazardous to work with. Insuring your hands are clean, or wearing disposable gloves, place parts in wire mesh baskets or hang with degreased black-iron wire (concrete section of hardware store).

Immerse parts in the optional pre-treatment (rinse conditioner – usually a titanium dioxide solution) per manufacturer’s instructions – a few seconds is fine. Do not rinse – put directly into the phosphate bath.

The phosphate bath is best heated by an industrial immersion heater. This is very expensive. Heating over flame or external electric element can cause hotter spots in the tank, which will cause a faster buildup of hardened sludge. For low volume, it may not be cost-effective to use an immersion heater. Also, a stainless steel tank will not react to the solution as much as a carbon-steel tank. However once a layer of phosphate has built up on the surface, it becomes non-reactive. If using a stainless pot, make sure the rivets holding the handles are not aluminium. Over time, aluminum rivets will dissolve, causing the handles to fall off.

Bath temperatures are going to depend on the particular brand of solution, coating thickness desired, and immersion time. This is where the technical side of phosphating far exceeds the requirements of the home gunsmith. Typically, the hotter the solution and the longer the immersion time, the heavier the coating. However too high a temperature will increase the amount of phosphate expended that does not adhere to the surface of the metal, but falls away as flock. You must not allow the solution to boil. I have used a variety of brands, and find that 180 F is a good starting point. Raise the temperature (max 200 F) or increase the immersion time. It is of no value to keep a part in a tank after the gassing has stopped, as no further reaction will occur, so my standard is 15 minutes, or until the gassing stops, whichever comes first.

Remove from tank. Rinse in clean water. The hotter, the better, as it will dry faster. There may be a thin layer of white flock on the parts, which can be scrubbed off with a brush. For hollow parts, I keep a variety of long-handled round brushes available – some bottle brushes from a paint-supply store, and larger brushes used for cleaning vents. For rifle barrels, run a brush through the bore. On deep, blind holes like an AK-47 bolt carrier, you may be surprised to find some sand coming out with the rinse. Inspect the part. If there is a minor flaw, the specific area can be cleaned and blasted again, then the whole part returned to the solution. Only the blasted portion will undergo a reaction. If a large area is flawed, then it is best to blast the whole part and start again.

Using compressed air, blow all the parts completely dry. If phosphate is the final finish, dunk in water displacing oil like WD-40, hang to drip dry and/or wipe dry with paper towels (be mindful of the fire hazard inherent with oil soaked paper towels). If the parts are going to be painted, paint as soon as possible or store in a very dry area. The virgin material can get a fine dusting of rust if not oiled. Sometimes this dusting is caused by impurities in the rinse water oxidizing. I am fortunate to live in a dry environment. For those in high humidity areas, paint as soon as possible.

Phosphating Tips
After initial phosphate mixing per manufacturer’s instruction, replace water lost through evaporation. I have been unable to determine how much of the phosphate concentrate evaporates compared to be used in the process. My technique of alternating between plain water and water mixed with the phosphate concentrate has worked well to replace evaporation, which leads me to believe the water evaporates faster than the phosphate. If coating thickness seems to decrease over time, add more of the concentrate solution.

Typical phosphate solution concentrate is about $51 per gallon and enough to do at least 20 complete rifles. If an accurate measure of the reactivity of the solution is desired (total acid to free acid ratio), you will need to buy titrating equipment – see advanced techniques section. My experience has been that it is a waste of time. It is far less expensive to completely replace the solution than to determine how much of which chemical should be added to balance the tank.

An 18 gauge black iron tank from Brownells lasts me about 6 months of heavy use before the acidic solution eats through the tank. The life of the tank can be greatly increased by storing the phosphate solution in plastic buckets when not being used. Practice with handguns, magazines, or small parts, on the kitchen stove with a $20 stainless stock pot or porcelain canning pot. It is easier to learn with small containers and you have less up-front investment if you decide against continuing.

A new solution will not produce an appropriately thick coating until conditioned, either by processing non-important parts (such as those being painted), or the one-time addition of a little steel wool or iron powder in a cheese-cloth sack, usually in a low temperature tank for about an hour – see manufacturer’s suggestions for details.

Five gallon buckets make good rinse tanks for smaller parts. I use a 50 gallon utility sink as a rinse tank. Cold water is fine, but hot water dries faster.

If using a gas heater, keep the fans and your air-hose away from the flame as it will make it hard to maintain a consistent temperature. I blew out my flame several times before using a piece of roof-flashing to shield the flame.

A 40″ Brownells tank fits diagonally on many kitchen stoves. Keep stirred as the heating elements will not hat the tank evenly.

To do a small part, a glass cup or small Pyrex tray with enough solution to cover the part can be heated in the microwave. I’ve never had a submerged metal part cause arcing in the microwave oven, but you may wish to heat the solution first, then drop the part in. Keep covered with a paper plate to avoid splattering solution inside oven, if it boils. Clean cup well before using for food.
Safety Issues
Have excellent ventilation. The steam from the phosphate bath is slightly corrosive and can oxidize tools and even etch contact lenses with continuous exposure.

The floc is poisonous. The phosphate solution is poisonous. The phosphate concentrate is acidic and can burn you. The phosphate solution is acidic, but the lower concentration. Maintain adequate ventilation and wear gloves, goggles, and an apron. Follow the manufacturer’s specific precautions. I’ve fished parts out of a 140 F tank with bare hands to no ill effect, but people have different sensitivities to the acid. I have also spilled a large quantity on my pants and received minor burns – like a sunburn, which no amount of rinsing eased – baking soda cooled it off.

As long as your flocculent sludge is not settled on your parts, it will not affect the process. However it eventually should be filtered out. After cooling, the sludge will settle to the bottom and the clean solution can be siphoned off the top. I siphon through large coffee filters. Keep your wet sludge in an open bucket. The water will evaporate off and minimize the amount of hazardous waste that must be properly disposed.

When making a new tank, to avoid waste, save your old solution to add as a replacement for evaporation.

Color Changing
Old phosphated parts have a green tint to them that some people like to duplicate. There is much debate about what causes this green tint, and how to best duplicate it. One thought is that cleaning solvents over the years give it the tint. This may be because of the copper fouling that dissolves in the solvent, as copper oxidation is green. While the cleaning solvents may indeed cause a green tint, Another cause may be the chrome-based sealant used as a post-treatment. A zinc-phosphate plus a chrome-based sealant yields zinc chromate – a crystalized salt that is translucent green. Finally, my research has shown that in 1942 patent, Darsey of the Parker Rust Proof Company, added copper to the recipe to reduce the alkalinity requirement and operating temperatures. I suspect this copper also results in the green tint.

You can age your parts by baking on an oil. This is a dirty process that yields noxious fumes, so should not be done in the kitchen. Dip your gray part in a medium weight oil, like motor oil, or smear with a heavier grease. Dry thoroughly. Place part in a 400 F oven for 20 minutes. Inspect. Increase time if necessary, checking every 5-10 minutes. As the oil burns, it darkens the surface of the metal, aging it to a green-brown tint. Quaker State wheel bearing grease has a greenish tint. I use Castrol or Coastal amber wheel bearing grease for a nice patina, but I choose it because it duplicates the color of Cosmolene and I use it for other things around the shop.

Home-Made Solution
While you can mix your own chemicals and titrate for total-acid to free-acid ratio, I do not find it time or cost-effective. The concentrate solutions from DuLite or Brownells are convenient and more cost-effective. Even at $51/gallon + hazmat fee (Brownells current retail price for Manganese concentrate) you are looking at less than a dollar or two per rifle in solution cost. For those wanting to play around with their own chemicals, here are some recipes.

manganese phosphate formula (Richards)

  • 61 grains phosphoric acid 1.5 SG
  • 37 grains manganese dioxide
  • distilled water to total 1 quart

manganese phosphate formula (Parker)

  • 365 grains phosphoric acid 1.5 SG
  • 22 grains manganese dioxide
  • distilled water to total 1 quart

Testing Solution
The key to a successful bath is to maintain the total-acid to free-acid ratio at 4-6:1, with 5:1 ideal. You must measure the ratio, and then add acid or base to adjust the ratio. You will need:

  • 2 x 100 ml conical flasks
  • 10 ml graduated cylinder
  • graduated glass eye-dropper
  • normal 10 sodium hydroxide solution
  • phenolphthalein (reagent)
  • bromophenol blue (reagent)
  • 75% technical grade phosphoric acid
  • sodium hydroxide (caustic soda, or lye)

Titrating for total acid content: Put a 10ml sample of phosphate solution from your tank into the first 100mm flask Add 5 drops phenolphthalein to flask and swirl to mix. Add drops of N10 Sodium Hydroxide. Count the number of drops it takes to change the color to pink and hold the color for 15 seconds (May take 20-40 drops). The number of drops is your total acid content.

Titrating for free acid content: Put a 10ml sample of phosphate solution from your tank into the second 100mm flask. Add 5 drops of bromophenol blue and swirl to mix. Add drops of N10 Sodium Hydroxide until solution turns light blue. Number of drops is your free acid number.

Two burettes with stand is a more accurate measurement of “drops.” 1 drop = 1 ml. This test can be done with a dropper, but it has to be graduated, so that you can read what’s been added. Dispensing from a graduated dropper tube is tricky at best, and the risk of overshooting the end point color change is high.

Adjusting the free acid can be difficult with volumes of solution used in small scale phosphating processes. It takes only a small amount to affect the ratio. Too much caustic soda will neutralize the free acid, and cause the metal phosphates to drop out of solution. Adding more acid won’t put them back.

  • Example: #1, First flask, 27 drops to turn pink. Second flask, 9 drops to turn blue.
    Ratio total:free = 27:9 or 3:1. Add 75% technical grade phosphoric acid.

  • Example: #2, First flask, 28 drops to turn pink. Second flask, 4 drops to turn blue.
    Ratio total:free = 28:4 or 7:1. Add sodium hydroxide or sodium bicarbonate.

  • Example: #3, First flask, 30 drops to turn pink. Second flask, 6 drops to turn blue.
    Ratio total:free = 30:6 or 5:1. Phosphate solution is ideal.

Painting Tips
Speed is a function of economy of movement. I have probably painted 10,000 rifles over the last 20 years. There is nothing more frustrating than getting ready to reassemble and finding a light or missed spot that needs to be repainted. Like in blasting I developed a time-effective method of painting that requires a minimum of movements and obtains maximum coverage.

Hold in front of receiver and paint all inside surfaces through the magazine well, the rear trunion opening and top of receiver. Keep your paint volume a little low and your air pressure a little high. There are a lot of surfaces inside the receiver that need coverage, but but are not cosmetically important. Be particularly careful around edges to apply multiple light passes. A heavy pass will walk around the edge to the visible external surface.

move your grip to the space between the gas block and front sight and coat the area between the rear sight base and the gas block. Then hold the front sight vertical between to fingers and coat in front of the gas block. There is no need to go beyond 30 degrees from perpendicular with the spray gun, nor should you be concerned with total and even coverage at this time. You will get less runs if you have a minute for the paint to set before applying more.
Insert a hook through the front sight, spray the rest of the front sight surfaces (red area) that are lateral to the bore. Then hold the spray gun 45 degrees to the barreled action – far enough away so a drip cannot land on the action – and spray while turning. Your targets are all surfaces facing forward relative to the bore (white arrows).
Secure the receiver with a second hook under the rear trunion, flip the barreled action over and remove the first hook. Now, spray all the rearward facing (relative to the bore) surfaces and let it dry for a few minutes – long enough that it is not tacky to a paint-free glove.
holding under where the handguards would go, examine your work. If you note any light spots that are not on the the flat surfaces parallel to the bore (side of receiver, rear sight block, etc) touch them up. The make broad passes along the entire barreled action – this final step is to even out any slight differences in texture and hue that can be caused by varying the distance from the surface – closer = more gloss, further = more flat. Hang for a few minutes until it is dry to touch. I can do about 12 barreled actions in an hour. My Curing oven will only hold 8 at a time. I find it ironic that the step some people think is most important to the final result is actually the fastest and with a little practice, the easiest. The finished paint appearance is 90% dependent on the prep work and 10% on the spraying.
Painted AKs ready for reassembly.
Drip drying after dunking in oil.
Completed AKs ready for test firing and shipping.
Retro AR-15 with steel parts Parkerized, and aluminum parts painted with a gray color, to match the original Colt gray anodizing.
MP5 with a combination of Parkerizing and “Paint over Park”. Early painted finishes were too thick for close-tolerance parts, so even with modern finishes, masking is appropriate to maintain the original appearance.

Leave a Reply

© 2022, T. Mark Graham, Arizona Response Systems, LLC. All rights reserved.