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for the best results with any spray-on finish, surface preparation is the key to good looks and durabilty.
ideally, you should hose the rifle down in solvent...acetone (very low flash point! be carefull!!) is the best.
soaking the rifle is even better.
after the initial scrubbing, bake the rifle for an a half-hour at 150°-175° to get the oil/grease trapped between the steel parts to flow out. do not get the steel so hot as to cake/dry the oil deposits on the steel.
after baking, an while still warm (not hot), more solvent and elbow grease...this time using a higher flash point solvent such as brake cleaner or carb cleaner.
after drying, preheat the parts to 100° or so. do not get the surface dusty or oily. wear cotton gloves to hang the metal parts on coat hangers or wires and spray them down with multiple thin coats.
baking dry at low temps is never a bad idea.
allow to cure fully for at least one week, better yet, 2 to 3 weeks prior to reassembly.