@chris a posted:My recommendation would be to try "red" Scotch Brite pad to try and remove the darker blotchy areas. I'd do it dry first attempt and keep a damp paper towel handy so you can wipe the area and see if you're making any progress.
The wet sanding could work, but you run the risk IMHO of burning through the red paint on any high spot or corner.
You could add dish detergent if it's stubborn. The Scotch Brite pad if you work from the top (crown) of the roof down should leave you a surface with a little "tooth" that you can go back and apply Pan Pastel or weathering powders to blend it in. If you don't have any Pan Pastels, you could experiment with some lightly diluted acrylic washes in Burnt Umber/Grimy Black. Key word here is lightly diluted, very little craft water based paint, lots of water.
Chris A Your suggestion nailed it; once I realized I couldn’t get scotch bright red in a grocery store. LOL. I ever so lightly moved the pad and then wiped it with the damp towel. I did it 3x and got to the level I was looking for. As I had said the rest of the engine and trucks looked good to me. So thanks to you I have my weathered piece. 
