[Pontiac1976]

First I would clean with warm water light car wash ( soap) med to soft tooth brush. Keep it wet clean brush often. Dry use wax polish remover. Use blue shop towel keep turning to a clean area often wipe if a little wet dry with another towel in your other hand.
Well it depends on how deep the scratches on the grill fins or how much rot on the plastic do any work need just take the sheen off.
Spray down fins with water lightly sand 400 wet/dry paper if there still heavy scratches in the paint, if most of the scratches are very light sand with 800 wet/dry paper make sure it wet water down. (spray bottle or bucket water clean water the if it gets to milky)
When finished hose down with water let dry or blow dry.
Wipe down with SEM Products, and SEM soap cleaner, SEM wipes clean patches, #39863 SEM Plastic Adhesion Promoter and fallow there direction on all there SEM Products. Then mask off the grill what needs to be painted different color start with inside first and or dark color first, then lighter color outer side area last mask. After all most hard to dry pull your masking tape off let dry. Then the same for the second color area painted dry. Do your clear let dry. fallow there direction painting time temp drying time as close with taking in humidity in count. Let dry clean area, or bake in hot sun few hour turning every 1/2 for the cool the fins a little then put in side for a day or two or on truck. Do not wash do not use any harsh cleaners or soaps 60day then only mild clears soaps.