All of the gun bluing that I've seen works only on steel but I recently tried Birchwood Casey's 'Aluminum Black' with mixed results.
I've been meaning to do a full writeup but haven't gotten around to it.
The product is a light blue, very runny liquid with no noticeable odor.
DANGER: POISON is in petty bold lettering so this might not be a product for some of our more junior members.
As far as I can tell it’s just an ‘instant tarnish’ kind of product.
The instructions call for the AL to be cleaned, degreased, and brightened with steel wool.
After application you’re supposed to wait 1 minute and then rinse with cold water.
I used it in the situation Don shows here (Don's pics, not mine).
Courtesy of Nascar, I had a similar E1 body with the trimmed lip for Arc compatibility and I used the Aluminum Black to darken the exposed cut.
The milled aluminum was VERY smooth and I applied the AB with a Q-Tip.
I didn't need to 'paint' it on -- it just flowed around the smooth aluminum.
The end result was a very dark matte gray.
Not a real good match to the HA-III but (for me) better than the raw aluminum.
(I believe Don's pic above was done with a black Sharpie.)
Interestingly the color would not extend all the way to the anodized areas and there was still a hairline crack of raw aluminum showing at the edge of the HA-III anodized area.
That area doesn't get a lot of wear to begin with so I can't comment on wearability but after 2 months it does still look the same.
I also tried touching up an old black Arc-AAA that had the raw AL showing at the tips of the knurls.
The form factor here allowed for dunking the whole body which is what I did.
After the first dunk I didn’t notice a lot of improvement so I re-dunked – and re-dunked.
My guess is that the AB works fine on raw aluminum but not on itself.
The Arc began to smoke and produced a lot of sulphury smelling fumes.
The liquid bubbled, foamed, turned a metallic black color and took on the same sulphury odor.
The Arc came out a very uneven medium dark black but the surface was crusty similar to actual rust.
The label promises no dimensional change but that must be on raw AL only.
Once the AB is applied, any future applications seem to just build upon themselves in an uneven and unstable crust.
The dark crust would come off on my hands and the color would bleed onto whatever it contacted..
It needed repeat washings before it stopped bleeding and flaking. And by then I was back where I’d started.
I also tried a stripped and bead-blasted CMG Infinity.
Similar to the Arc, the first dunk produced only a light blue/gray.
Repeated dunks produced the same crusty finish as the Arc.
This time after multiple washings and even a not so mild sandpapering the Infinity still shows a pretty dark black in the pits of the bead-blast.
In every test I skipped the cleaning, degreasing, and steel wool which probably made a huge difference but I still feel this is a product for only very smooth milled aluminum.
The cold blue is just for iron. It just oxidizes the surface. Beautiful finish on carbon steel. Never tried it on stainless. I've tried the brass black on brass but wasn't too impressed. It difficult to get an even finish and it comes off too easily. I'd imagine the aluminum one is more like the brass black [img]/ubbthreads/images/graemlins/confused.gif[/img]
I used the BirchWood Casey products years ago. As I remember it work pretty good on a flat surface but on a corner or sharp point it didn't take because a bead of liquid won't stay on a sharp corner. So any radius might be a problem. Unless the stuff has improved, no cold bluing ever last very long.
I used some aluminum black a few years ago. I don't remember if it worked better hot or room temp. The part warm/hot that is.
It probably wouldn't darken close to the andodized/bare metal tranistion since anodizing is a penetrating finish. I think the place that HA's some of our parts claims one thousandth penetration plus one buildup for a standard immersion time. I used it on a triggerguard on a Black Powder pistol and it still is black after all these years. Or rather a darkish matte grey.