OK guys, here's the answer to a couple of questions... Somebody asked where you use Tiger Hair and what about moisture getting through from the backside...

Most body filler is polyester resin plastic (just like fiberglass resin) with a talc filler added. Yes, the talc is the same stuff that is found in baby powder and it therefore absorbs and holds water. The "lightweight" body fillers generally have some of the talc replaced by microscopic glass beads wich makes it only slightly less absorbent, but improves sandability. Tiger hair, Kitty Hair, Dynaglass, Dynahair and other brand specific names designate fillers that contain no talc, but have chopped up or ground up fiberglass particles instead. These fillers will not absorb water and therefore would be used wherever filling holes that might be exposed to the elements (water, mud, condensation) on the backside. Different ones also have different length glass fibers in them. The long-fibered fillers are used for holes larger than 1 inch in diameter; short or powdered fibers everywhere else.

So why not just use these types of fillers exclusively? First, they are not as easy to work with because they tend to be harder when cured. Second, they tend to be considerably more expensive than conventional filler. Third, since they are "resin rich" they may cause discoloration of the topcoats later, so always finish them off with a surface coat of good conventional filler. Also, the long-stranded versions of these fillers may tend to pop some of their fibers through the paint months after the work is finished if they are not buried under a layer of conventional filler.

Hope this helps...