Birchwood Casey puts out some 'touchup' paint, one for steel & one for aluminum. I believe you can get it blue or black & you can get it in a bottle or a touchup pen similar to a felt marker pen. It does not really blend in real well & can be easily seen as a touchup job. But it does protect from rust. There are DIY coatings where you recoat the whole pistol, but I'm not familiar with them.