I recently finished a home made AR-15 receiver made from 304 SS and
6061-T6 AL. Now I want to give it a cheap flat black or dark gray finish, preferably something I can spray on and maybe bake in the oven. I remember a discussion something like this a while ago, but I can't locate it.
Any ideas or suggestions, either for the process or locating the discussion?
Oops, I missed the alloy callouts. Never mind. I've heard of aluminum blackening finishes but know nothing about them. Stainless can be blackened with black oxide, but if your receiver alloys can't be separated, I don't know if the pickling would affect the AL.
I think I'll back out of this discussion now. I'm in over my head. :-/
-- Some people hear voices. Some see invisible people. Others have no imagination whatsoever.
Black oxide is available for stainless, but it leaves a thin coating that's not tough at all. Black nickel is better but it's really gray, not black. Black chrome is best but also the most expensive. You'll have to send out to have those finishes put on but you can do the black oxide with a Caswell kit. If you want the receiver to look bright for a long time, black chrome is the trick. You may have seen that finish on good handguns.
There are lots of ways to blacken aluminum. Commercial treatments use alkaline or acid treatments. Most of them are gray, too, and you won't get a good match with the stainless.
You can have the aluminum parts anodized and treated with black dye. The old dye used for this was black India ink. It doesn't sound like much, but the anodized treatment is tougher than the alkaline and acid treatments. You can't do this with hard anodizing, which really is only thick anodozing, it's not any harder. It has to be a standard, thin anodize and you have to dye it right away, while the anodizing is still porous. It seals itself up in humid weather, and it happens in days, or even hours, and then it won't take dye. You should send out to a jobber for this, too, unless you want to experiment and you know how to do anodizing.
If you want a uniform color on the stainless and the aluminum, powdercoat is the way to go. It's not especially tough, but it's cheap and it can look good if you don't beat your guns around or let them bang into hard things. I never heard of a bake-on spray paint, except for muffler and manifold paint. I don't think that would look good. Bake-on barbecue paint I've seen is flat black.
When Remington first started making stainless shotgun barrels, in the sixties, they had them iron plated and then hot tank blued. It was a really good finish. It was expensive to do in small batches.
I hate color finishing stainless because it's hard for the small-time operator to get a smooth and even job. I used to blue guns as a sideline. I'm not an expert. I only applied hot-tank and cold-blue finishes. A real cold blue, sometimes called rust bluing, can be beautiful, but I never heard of a process that works on stainless. Check with Brownells to see if they make such a thing. Sometimes it comes out purple, anyway, on alloy steels.
You're asking for a relatively hi-tech finish and they don't come cheap.
I've used Norrell's bake-on resin paint and have had good luck with it. I used multiple coats, baked in a toaster oven between coats and applied with a cheap airbrush. Some of his smaller quantities are cheaper than a can of the Brownell's, but you do need an airbrush.
Can't parkerize aluminum! The military finish is hard anodizing, not the same as decorative anodizing, uses chromic acid. Best bet is DuraCoat or Alumihyde, see Brownell's website. Cheap won't last. Midway may have something, too. If you just want cheap, look up some flat black $5/can at a big box. It'll chip and you'll have to strip and repaint in a year or two, but it's cheap!
Get some 400 grit silicon carbide lapping paste and lap all the surfaces with it, then clean them. This'll give it a very good key for the paint. Then paint with the lacquer/Alkyd-auto-paint of your choice.
Of course the correct finish would be black crackle :-)
Many year ago we used a Sherwin Williams rattle can paint to paint staples that were used in a air driven stapler. These were 1.25 or
1.5 inch staples. The paint was a one part epoxy appliance paint. We got the tip on the paint from the Duofast staple salesman. That was eons ago, so recommend that you contact the Duofast salesman in your area and ask him. Sherwin Williams may have changed the formulation, or there might be something better by now.