advice on bonding fiber glass to metal, a spoiler to my trunk lid Options

[brant]

So years ago I ran a trunk lid with a rayco spoiler
the (poorly trained) body guy that did the work in 1986 simply riveted it to the trunk panel and then used bondo to hide the seam.

Of course every 3 months the bondo would crack.
I probably had him re-do it 3 times before I just gave up and left the crack.

I later changed the trunk lid out and put the old one in storage.

I was thinking today that I could probably strip it and bond it CORRECTLY and then re-use that lid in my current project.

So the question is: What are the proper bonding agents to use when bonding fiberglass to metal.

The rayco spoiler is at least gel coated, almost plastic like and not raw fiberglass.

I do want to hide the seam again after I get it correctly bonded. are there special fillers that will beter take the abuse, or will regular old lightweight polyester filler work as long as its bonded with something better than 6 rivets?

what do people use on fender flares.
I've seen nice fender flares that lasted for years without cracking.

thanks guys.
brant