Personally I just let them pick up wear as I use them. However I paint guns (and lots of other stuff) to order. I've been painting models for 30 years now and have specialised in weathering, I do lovely rat look models of people's cars, well clients love them anyway.
I have done quite a few RiFs now but as I tell buyers, if you buy a pre weathered gun you need to manage your expectations, weathering if painted on will come.off if not looked after properly.
Even the most protected paint job can be chipped an worn and will contrast.
If you 2ant a gun weathering then hit me up!! ?