What is the correct way to prepare fiberglass bumper fillers for painting?

I purchased aftermarket fiberglass bumper fillers to replace the dried and cracked plastic ones on my 1985 Buick Riviera. What is the correct way to prep them for painting (sanding, priming, etc.)? The fiberglass fillers are black. The car is a dark metallic blue.|||Sand with 220 grit sandpaper to get shine off then paint. Primer should not be needed at all unless you sand too much and you start seing glass fibers.