Repairing/modifying a gelcoated part?

[69427]

I've had a spare (fiberglass) rear bumper for the '84 for a bunch of years, waiting around to eventually 'glass in a spoiler onto it. I picked up one of those wrap-around, bolt-on spoilers that Chevy sold for C4s twenty years ago. This thing has a thick gel coat on it, but I wanted to do a bit of cutting and 'glassing on it to blend in with the 'glass bumper. I'm not used to dealing with gelcoated parts. Can someone give me a few hints on what to do here?

Thanks,
Mike

 

[SuperBuickGuy]

I'm a rank amateur, but got some good bits of advice off here about how to install the scoop on my car.... still, the best fiberglass how-to has to be this guy's http://vettemod.com/forum/showthread.php?t=366

 

[Twin_Turbo]

If it's just to get the contours correct without adding a bunch of body filler, you can rough up the gelcoat with grit 60 or so and glass over it. Helcoat is nothing more than polyester resin with a dye added. If it's structural you will have to sand down to bare glass for the best result. If you look in the 63 resto thread you can see examples of both ways the panels are all structural and sanded down to bare glass, the headlamp bucked just has lay up on the gelcoat