Tuesday, July 22, 2014

Techy Tuesday - Metal Finishing in the Small Shop

A post over at Theredneckengineer got me thinking about this topic again.  When you're done cutting metal, now what?  Steel, except for stainless, will rust quickly in all but the driest climates.  Aluminum won't rust, but it oxidizes and will develop a kind of cruddy, blotched look.

This has been a hot topic in the gun industry since before Sam Colt was a little boy.  A lot of finishes exist; from Parkerizing, and bluing, to Cerakote and the dozens of other finishes.  I'm going to focus on two simple processes: powder coating (because it's the one Redneck Engineer wrote about) and anodizing.

Powder coating is a painting process, and can be done to almost any material, like most painting.  You can powder coat any metal: steel pieces or aluminum; titanium or brass.  What's neat about the process is that you attach a ground wire to the piece to be coated and spray a powder that gets electrically charged in the gun.  Instead of the overspray of paint that plagues spraying liquids, the powder is electrically attracted to the grounded piece.  There's less over spray, and any overspray that happens is a powder that can be brushed off.  Powder that goes beyond that part will actually turn around and come back to coat the other side.  That's not to say you may not get better results turning the part, just emphasizing that attraction of the powder to the charged surface.  Once the part is coated, it's then baked in an oven, where the powder turns into a liquid and flows over the surface.  The temperature depends on the type of powder, but is around 300-400F.  The result is a hard, chip resistant finish that holds up better than the spray paints I've been able to use. 

Powder coating is new technology, but hardly exotic.  Powder coating sets can be bought at Sears, Harbor Freight and many others.  Two excellent sources are Caswell Plating and Eastwood; both of them are also excellent suppliers for anodizing. 

Anodizing is an electrolytic surface treatment process that is most often done to aluminum.  Compared to painting, it allows a more precise control of the finish thickness and typically looks better in a thin finish than powder coat tends to look.  It's rather more involved than powder coating, but a good finish in almost any color imaginable is within the reach of determined folks.  The word comes from using the material you're finishing as the anode (more positive piece) in an electrical circuit you make.  Anodizing is often referred to by its "type" (these come from MilSpecs) , and you may have seen references to Type III anodizing as a selling feature for gun parts.  Home anodizing is most often Type II, Low Current Density.  There are good, detailed explanations on the web, but I find my favorite is gone tonight (haven't checked it for a while).  This guy seems to have a couple of pages of his results and looks good to follow; but let me summarize here.

You may not have noticed the first important thing that was up there in the descriptions.  Powder coating applies a layer of paint over the surface, hiding it to at least some degree.  Anodizing is a surface treatment; that means it doesn't really cover the surface.  However pretty your surface is before you start, that's how pretty it's going to be when you're done.  The first step is to finish the surface until it is free of defects. Finishing metal surfaces is simple in concept, just tedious and time consuming.  You sand with progressively finer abrasives and either leave a matte surface (600 grit on metal looks like a matte finish) or proceed to polish.  Books have been written in place of that sentence, and industry has invested tons of money in getting pretty finishes faster.

The piece is then thoroughly cleaned and degreased.   After completely cleaning it, so that water doesn't bead up on the aluminum at all, it's time to actually anodize.  The work is submerged in a mix of sulfuric acid and distilled water.  Yes, there's a specific ratio; no, I'm not going to tell you what it is.  This piece is an overview, not instructions.  Two electrodes go into the acid: the cathode goes to the negative side of your power supply while the piece you're finishing is the anode.  The process depends on current, not voltage, so low voltage, high current supplies are usually used; the 12V, 20 to 30A supplies that hams use aren't adjustable enough although they're in the ballpark for capacity.  You want a constant current supply with adjustable current.  The actual anodizing part of the process takes a couple of hours, depending on how big the part is and how thick the layer you're striving for.

At this point, the parts have barely changed color, and they're coated with a thin, new coating of aluminum oxide (you can call it corrosion, white sapphire, or anodizing).  The guys who do this refer to the finish having pores that are open.  The parts are washed in distilled water, before going into the dying process - if you want to add color. The process is simple: heat a water solution of the dye (RIT clothes dyes are often used) up to about 140F, and suspend the part in it for a few minutes, checking to see if it has the color you want.  The dying process is self limiting and the parts just can't keep getting darker after some time.  If you don't dye the part, you go straight to the next step. 

The final step sets the color permanently.  You take the dyed parts and submerge them in boiling water for a half hour.  This doesn't need to be distilled water.  That sets the finish and results in the hard, colorful finish anodized parts are known for.  That is, parts not intended to be tactical black. 
The paintball guys are big on wild anodizing colors.  Hard to imagine someone making an AR lower in this color scheme.

Anodizing can also be done on titanium, and the colors are obtained by allowing different thicknesses of the oxide layer to build up.  Check out these titanium chopsticks, although the technique is commonly used in titanium jewelry.

This is admittedly a brief overview.  Not enough detail to begin either technique but hopefully enough to tickle your interest and get you looking.


7 comments:

  1. Both processes DO work, and both can be botched to hell and gone very easily... It takes practice to do it right, and many today don't or won't take the time to actually gain the skill needed... sigh

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  2. Great post Si! I come here to learn.

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  3. And don't forget that the energy and water saving appliance that leaves dishes dirty will strip that anodizing right off....

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  4. Thanks for the linky love, SG.
    You went into a lot more depth than I did, and, as usual, wrote a very interesting article.

    The good thing about having an oven to bake powder coat is that it can also bake on cerakote, and if it will get hot enough, anneal hardened metals.

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  5. Redneck - want to do a piece on cerakote? I've never even looked into it.

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  6. Actually, I'd love to.

    First.....I gotta buy some. =(

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  7. Agree with NFO!

    I've seen powder coating that's damn near impossible to remove, and some that just falls off in six months or less.

    A lot of it has to do with cleanliness and surface prep, as anybody who's ever done any serious painting understands.

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