What finish can you suggest or recommend for an old vietnam vintage A1 lower receiver, DURACOAT or just have it re-anodized? I believe anodizing "eats" up the surface of the aluminum, and can make it thinner by a few thousand of an inch. Which should give better protective properties for the aluminum? thanks.

Duracoat or gray GunKote.

www.moly-resin.com

"GRAYISH-BLACK Flat     Dark gray-black coloration with a flat appearance. This duplicates the appearance of the early original Colt AR-15/M-16 finish. This is the same product that is purchased from us by the U.S. Military to refinish Colt M-16's."

This stuff is very easy to use.  In the Build It Yourself section find Stickman's Refinishing thread.  I did three AR's in the last few days, OD and black.  This stuff is easy to use, just follow Stick's directions.

The finish is as hard as, or harder than the original anodized finish.  It goes on very thin and does not fill in stamped in markings.  On the barrel, you can still see the machine marks where the barrel was turned down.

DuraCoat sticks to anodized aluminum way better than GunKote.

It depends.

Anodization is a ceramic made of aluminum oxide which is very hard (which is why it is used in sandpaper and abrasives) and is rated close to diamond in hardness. Sulfuric acid used in the process will dissolve the surface, but if done properly, the dissolution is offset by the growth of the new anodized layer. An electro-chemical process converts the surface from aluminum into aluminum oxide, which actually grows from the surface as a crystalline structure.

What does more to dissolve or thin the aluminum is the removing of the old anodized surface by abrasive blasting, sanding or chemical stripping, rather than the anodization process itself. A general rule of thumb is that a lower/upper should only be re-anodized 1-2 times as the removal of the old surface will increase hole sizes.  Additional re-anodization can be done if slightly oversized FCG pins are used.

Anodization is a much harder than anything that can be sprayed on the surface of aluminum. The anodization not only grows outward from the surface, but half of its thickness actually penetrates the surface of the raw aluminum. Duracoat, Norrell's and other coatings are great in providing a lubricating coat and corrosion resistance, but they don't approach the hardness of an anodized surface.

Regarding the 'it depends' statement. It depends on what condition the lower is in and what you are willing to spend, what the lower is worth to you, and what you are looking to get for a result. If it is just an appearance issue and the anodization is in good condition, duracoat, norrells, etc. are great. If you are looking to restore it to original condition and provide long term protection, then anodizing is the way to go.

If your looking for the ultimate in hardness and durability and the current anodization is in bad shape, have it  stripped, Type-III anodized, not dyed, not sealed and specify a 2mil thickness. Then coat it with duracoat or norrell's to add an additional layer of protection and lubricity to it, and will provide the color it (its also easier to touch up scratches).  Proper Type-III anodizing does not dye well so don't bother paying to color it. If done correctly, it will come out a light/med gray color. Sealing the aluminum isn't required since you aren't dyeing it and actually reduces the hardness of the anodization by around 25-30%.

Around me, it costs about $75 per lot for Type-III, un-dyed, un-sealed anodizing of raw aluminum. It should only cost a few $$ for each additional piece done at the same time. If they are going to strip it, then add a few bucks per piece.