Parkerizing can be avoided, especially on non-heated metal parts like a folding stock. They can be refinished using high grade automotive paint products.
If you are refinishing a sandblasted piece, blow down the parts carefully with compressed air BEFORE applying any type of surface cleaner to the part. This will avoid dust from building in all the nooks and cranny's. Initially clean the part with Alcohol: plain old store bought Isopropryl Alcohol. It evaporates very quickly yet is strong enough to lift most types of oil and contaminates from the piece.
You will need an undercoat that bonds to the metal. You have 2 choices: an etching type of primer (acid based that will literally 'burn' into the metal, or an catalyzed expoxy. I prefer a catalyzed Epoxy like DuPont's 2690S "Prime & Seal" for this. First off it is available in black and it is so durable it can be used as a final coat if desired. The Epoxy dries to an attractive semi-gloss and will withstand most solvents such as laquer thinner, parts solvent, WD40 and so on, as well as stuff like Gasoline and Acetone. Once fully cured, the Epoxy primer cross-links to a waterproof surface. No moisture will penetrate it. The primer itself is very flexible as far as paints are concerned and very VERY chip resistant as a result. I have used this type of primer on engine blocks and intake manifolds as well though it is not recommended by the manefacturer.
If you are unable to Parkerize any of your parts, using a good 2K Epoxy primer on your parts is your next best bet.