Trying to get my '69 Barracuda ready for the track this spring. The doors back is original sheet metal the front end is fiberglass. I have ~1/2" gap in between the fenders and the doors so I need to build up the edge of the fenders. What should I use, something like a kitty hair filler or something specific to fiberglass?

Also, going cheap on this car but was wondering would you use Rage Gold filler ($58) or "Bondo" brand filler ($20) for the finishing work? I've always used Rage.

For a 1/2" I would just use light weight roll mat
fiberglass and resin, 2" wide... either filler product

x2 ....but use some GOOD

I agree use fiberglass mat.. The way I would do it is first grind the backside of the fender edge to roughen it & expose some matting to promote a good bond... Then use thin cardboard like a cereal box is made of...First I use clear packing tape to seal the cardboard... Then lay it over the front face of the fender & mark the shape on the cardboard, cut along the line then tape the cardboard so it's edge is butted to the fender edge, this will create a small step the thickness of the cardboard in the soon to be laid new fiberglass, I'll explain why later.. Now working from the backside, cut a 1.5" wide strip of fiberglass mat, not cloth, mat... mix up some resin & activator drip the strip of fiberglass cloth in the resin & brush a layer of resin on the cardboard mold & the backside of the fender.. Then lay the fiberglass cloth & brush it into place, let it set up a few hours then apply a second layer of cloth to the back side, same deal soak the cloth in resin & brush some on the area before applying the cloth.... let it set up for a day, strip the cardboard then brush a good heavy layer of resin on the front side to fill the void the cardboard left.. This method makes sure no cloth protrudes from the finished fender surface.. You can build the fiberglass resin up to the point no plastic filler is needed..

Make sure to grind and bevel the edge from both sides,before laying up the cloth,so it doesn't show the ghost line in the finish later.Try to avoid using resin to build shape or fill voids.Without fibers,it becomes very brittle and will chip easy.I would recommend using epoxy resin,you can pick it up at any marine store.Use chopped strand cloth,not weave.If you have voids and low spots, you can fold the cloth a few times and cut it up,stir it up in resin and you have solid filler. Z-Grip is a good filler that doesn't cost $50,it's around $25-28 per gal.

Thanks for the help everyone.

I'll give it a try!