I have used Bondo Glass for similar projects in the past, it is basically body filler with fiberglass strands pre-mixed in. (far less of a pain compared to mixing in fiberglass with regular bondo, and the fiberglass is consistent throughout the filler) I believe they have a long strand and a short strand formula.(bondo glass is short and bondo hair is long) The short is a little easier to work with.
I kept the layers about 1/8" thick, but never more than 1/4", because if you go too thick it will harden on the outside before the inside cures, and then shrink as time goes on and ruin the paint and become a serious pain in the ass. It only takes about 30-60 minutes to completely cure depending on temp and humidity, though letting it sit overnight before you add the skim of regular bondo isn't a bad idea. Let each layer harden for 20-30 minutes before you add another, that way you reduce the risk of shrinkage. You can shape and mold it for a few minutes before it starts to set to save on sanding, and use 80 grit for rough shaping when it gets hard so it doesn't take forever to shape. Then once you are really close to the desired shape, give it a skim coat of straight bondo to remove any imperfections and sand with 400 and then 600 to get a nice smooth finish. You may need more than one coat, as you want to make the surface as smooth as possible, as it's hard to fill voids with primer. Apply several coats (I use at least 5, sometimes more) of a primer/sealer, as it will soak up paint like no tomorrow if not sealed, and that gets to suck real fast. Sand the primer between coats with 600 grit so all the small imperfections get filled, but make sure you have complete coverage when you are done. If you want you can progress to 1000 grit, but 600 will usually give good results.
If done properly, you will have a super smooth surface ready to be painted. Be sure not to use spot putty, as the stuff never dries and just sucks really bad. It looks convenient, but is anything but.
I learned this through trial and error doing custom speaker enclosures, so I have it worked out pretty good. If done properly it will be rock hard and super durable. Just be sure to use enough hardener and mix it thoroughly, folding it into the filler so as not to create a ton of air bubbles.
It seems like more work than it really is, and goes pretty fast once you get moving.
link to Bondo Glass:http://www.jamestowndistributors.com/userportal/show_product.do?pid=6102&familyName=Bondo+Glass+Filler
Bondo Ultimate works good for the final layers. Lays down smooth.
Also be sure to rough up the area really good with 80 grit prior to applying the bondo glass so it has a nice rough surface to adhere to. The rougher the better.
Hope this helps. Feel free to ask any questions you have, I will do my best to answer.
Kenny
