What can you use to build up over gel coat?

If I need to fill in a low spot of lets say a 1/8-1/4 inch is it better to use bondo or epoxy resin with that powder filler added? I guess the other option is to lay glass over the gel coat or sand off the gel coat and then lay glass which sound like way more work.

Bondo will not stick! Use that 3 part West Marine stuff. Scuff up old gel coat real good to give tooth for good adhesion.

Bondo (if anyone would ever actually use that brand) will indeed stick to well abraded gelcoat.

If I were you I would stay with a polyester resin based product, like Bondo. If you are going to do a thick application then you need to consider that Bondo can be brittle and could crack if the panel its attached to is flexed. You might want to consider "reinforced Bondo" like Evercoat Kitty Hair (http://www.evercoat.com/productDetail.aspx?pID=38 )if building up to 1/4", and then smearing with a final topper of smooth Bondo.

I have quite a bit of experience with epoxy resin based products (West, Tap Plastics, etc.) and they are always an absolute first choice -- when appropriate. However I don't think they are appropriate in your case. Polyester resin and epoxy resins don't play well together. Epoxy will stick to polyester, but polyester won't stick to epoxy. So once you put epoxy anywhere you can't come back over it again with polyester.

Stick with the polyester resin based products. They are faster and easier to use than the epoxy products, and will probably produce better results. Just make sure you seriously abrade the gel coat first (like 40 grit!).

Like any bodywork you must abraid the surface to make anything stick good. I like the fiberglass gelly for what your going to do. but a void going over 3/8 thick. If you need that muck better to re fiberglass the area to build it up

Depending on the size of your project, specifically the depth of the repair, this may or may not be the best thing for you. I like it because it is a one part product, and for me no mixing = less waste. That said, it doesn't cure fully if applied too thick. If filling a shallow low spot, this is the can I would reach for:

http://www.jamestowndistributors.com/userportal/show_product.do?pid=4154&familyName=Interlux+Surfacing+Putty