Re: Corrosion resistance??
I've used John Norrell's Moly-Resin, Wheeler Engineering Cerama-coat and home parkerizing. The parkerizing requires a bead-blasted finish and a tank large enough for your largest part (the barrel usually) ( You can do a handgun on the stove in a stainless or pyrex pan.) and a heat source to cook it for a few minutes at about 170 degreees. Don't do too many on the stove as the fumes will do bad things to your walls. And the fumes are poison so don't breathe them. You will see a lot of sites/people who cook theirs a lot hotter but if you keep the solution under 175 degrees, you can re-use it many times.
I've always used phosphoric acid and water but you can also add manganese dioxide and steel wool.
The Moly-Resin requires a sandblasted finish for best adherence but it isn't necessary, an air-brush and an oven large enough for your largest piece. The ones I can't do in the kitchen oven, I do in one I made from an old water heater and some electric oven elements from the appliance store and a thermometer.
The Cerama-Coat comes in a spray can and only requires an oven. They say you don't even need to sand or bead blast the parts first but I do anyway.
All of the above finishes are superior to bluing as far as corrosion resistance.
The Cerama-Coat gives a matte or flat black that looks very good and can also be polished to a sheen.
The Moly-Resin comes in many colors. Flat black, gray, od green, tan, brown, white. semi gloss black, high gloss black and stainless steel.
Parkerizing gives you a nice gray that gets darker the longer you leave it in the tank. (The guys that use manganese dioxide and steel wool and high heat cook theirs until it stops gassing. and I'm not real sure of the final color but I'm going to do one like that next time and see for myself.)