shady brownells sells some great great bake on finishes that are epoxy spray type.. called gun-kote... think they have another line of spray on bake on finishes too but cant remember
depending on the part valve cover wrinkle paint is nice especially if you use a heat gun for metal or a hair dryer for plastic parts.... but it take a week or more for it to really harden up to withstand constant touching.....
and I know you said no to bluing but there ARE some great cold blues out there but they are usualy made by some smaller companies and you gotta really look for it
both this
http://www.shootersolutions.com/gunbluingkit.htmland this
http://www.bluewonder.us/re both very good products!
brownells also has some nice case hardening salts where you litterally bed the item into the salts and heat them to temps... think its arounf 450-500 degrees and gives a case hardened look..... though I would NO bake that stuff in your regular oven!!!