Here is the way top gun winners weather their planes. First your plane must be painted with a two part paint system that after the paint cures it is unaffected by its own solvent. Most automobile paint fall in this category. LET YOUR PAINT COMPLETELY DRY. Now mix a small amount of dark brown paint (color of motor oil mixed with dirt) . start with wings and spray a mist on one wing panel let it set for few minute then get some paper towel folded into a small square dampened with your paint system solvent and start taking off the paint in a single straight wipes chord wise . what this does leave a small amount of paint behind the rivets and panel lines etc. Do the same for fuse but this time it is from top to bottom wiping motion simulating left in the rain effect. My be good for your next project