I'm not GB but someone who routinely paints rifles!
Not being one for protocols, I usually tape over what doesn't need painting and have at it with an assortment of RustOLeum colors! It's worked thus far!
I buy rifles to use, rather than worry about resale value - which is why I own Savages, not Merkels or Krieghoffs. Not that those aren't wonderful guns, it's just that a clod like me has no business with 'em.
It feels like sacrilege when you're standing there holding a can of spray paint, but ..... it really isn't that skeery.
Redial