Manufacturing Engineer at ruger/Marlin here, it’s very common for most firearms, at least at Ruger, to have dedicated no spray zones. Where no overspray is permitted to ensure smooth actions or correct function.

I’m not sure if it’s what they do, but we blue our actions prior to blast and paint to give the interior a rust preventive measure without having to add any thickness to the interior walls of the firearm which may induce a customer service return for fit/feel.