rust bluing sounds interesting... can anyone explain the process in brief?
It's pretty simple:
1) Remove existing blueing
2) Draw file away any pits or deep scratches
3) Sand/polish to about 400 grit --- you can go higher, but rust bluing usually has a satin-like finish and doesn't benefit from an ultra polish. This step is where you have to take the most care to keep the surfaces flat, the edges sharp, and to avoid harming any stamping.
Up to this point, you've basically prepared the rifle in the same way that you'd have done for hot blueing.
4) Degrease the rifle very well. The parts should sheet any water. I like to wipe down with naptha and then soak the parts in hot water with Dawn detergent.
5) Apply the rust blueing solution per manufacturers instructions. This usually involves a clean cotton swab dampened with the solution and then gently and evenly wiped along the metal.
6) Place the parts in a protected, but open area where they will start to rust or use a rust box to accelerate the process. After a day, the parts will have a fine red rust on them.
7) Boil the parts for a minute or two in distilled/deionized water. This converts the red rust to black.
8) Gently card off the velvety black rust with degreased 0000 steel wool or a fine carding brush. This will leave a smooth, but slightly darkened surface to the metal.
9) Repeat steps 4-8 a half dozen or so times until the depth of colour is built up.
An express or belgian blue process is faster and achieves similar results on most metals. With express blueing, you degrease the parts, heat them in boiling distilled water, remove and dry the heated parts, apply the blueing solution, wait 5 or so minutes for rust to form, then boil and card. Repeat the cycles as needed to build the colour.
The most important steps to any blueing are metal prep and keeping the pieces free of grease/oil. It's a good idea to change the boiling water regularly to avoid contamination. Laurel Mountain Forge makes a slow rust blue that contains detergents that are supposed to cut through any risidual oil --- I have some, but haven't tried it as I've been 100% satisfied with the Radocy express blue.
Rust blueing has a lot of benefits for the hobbyist since, although the solutions are acidic, they are not as toxic/harmful as hot blueing salts. I don't know a lot about the history of BSA Martini's, but a lot of the older rifles had rust blued finishes, so the result may also be more authentic.
