I've painted several sets of fender flares with good results. Start by washing and scotchbriting the flare. Scotchbrite just enough to kill any glaze on the surface. Wipe down with a good paint precleaner. Do not use laquer thinner!
Alchohol is OK, but pre-cleano is better. Then wash with soap and water again. I use diswashing soap as it will remove wax and grease. Rinse well and dry. Next spray with adhesion promoter ( a friend of mine owns a paint supply store and refers to this as glue in a can). You can then use a primer/sealer and paint. If you use a basecoat/clearcoat urethane you do not need to add a flex additive. If you use enamel paint you do need flex additive, but it can make the color not match as well. I have only done this with smooth flares, my current Ranger has the rough texture and I don't think it would look as good with a high-gloss finish. I just painted mine black with the Krylon Fusion plastic paint, and so far it's holding up well.
Hope this helps.
John
