as was said, take them off You are gonna have to mask almost the whole truck unless you want overspray on the whole thing.
scuff them with a grey scuff pad. red ones are very harsh and they are meant for priming, not painting over. you can do it wet or dry. if you do it wet, put a little soap in the water.
Wash them off. Dry them very well. Use a wax and grease remover from a paint store if you can get it. Its cheap at about $6 for a spray can. Spray it on, wipe it around and with a clean paper towel, wipe it off while its still wet. Glass cleaner can work, but it can stain and streak with certain kinds of paints, so i would avoid it to be safe.
Im assuming you are gonna rattle can it. Spray the first coat very light. You just want a pisser coat on there to make sure you cleaned it right. Let it sit for a couple minutes than go with a second, slightly heavier coat. Let that sit for a few minutes and if there are no reactions, then you can go with a nice medium coat. Let that one sit and give it one more. Most paints require 2 or 3 coats. So just check the can.
Let it dry for a while, slap flares and bumper back on and be happy.
2009 Ranger Sport - 4.0L/Auto 3.55 Limited slip. Black. 2500K Nokya fogs, Vanish cover, PreKey Leveling kit and 4 cupholders!
1994 Mustang GT - 5spd
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