How do I prep/paint GT or Cobra ground effects?

Dont bead blast them that will warp the urethane. if they have good paint on them wet sand them with 600 and shot your product over it. if they are ****ty sand them down smooth, feather edge any spots that are down to the plastic and prime before paint. personally i havent used flex additive on ground effects for a stang, only on the bumpers. our gorund effects are heald on by a lot of bolts an braketry.

Are they painted by someone else, do they have factory paint on them, or are they an aftermarket item with gel coating or something on them?

If they were painted by someone else, I'd probably strip them down to bare plastic with a chemical stripper specifically for urethane plastic, and then start from there..........because you don't know what the other guy did to them.

If they are factory painted, just scuff them up like mustangjoe said. If they are new aftermarket items, talk to them.

You won't need a flex agent with todays good basecoats and clears. Just make sure you use the proper primer/sealers and adhesion promoters/plastic cleaners if recommended by the paint manufacturer.