One of the things that has sometimes stymied me is the right filler for resin. Putty-type fillers like the old green Squadron stuff, 3M for auto body work, the tube fillers from Tamiya and the like, as well as the "liquid" fillers like Mr. Surfacer and Tamiya Surface Primer (which acts as a liquid putty directly from the jar) all flake off of resin because they are not able to bond with the surface like they can with styrene. I have used ACC/super glue as a filler in the past, but you have to sand it at just the right time... too soon and it can "ball up" and come off as you sand and too late and it is harder than the resin, meaning that the materials are removed at different rates by sandpaper, leading to surface defects of a different order, which is what you are trying to eliminate by filling and sanding.
I recently mixed up some of the J-B Weld (pictured above) to use as a bonding agent. It mixes like a two-part epoxy, where you stir equal amounts and then apply. While it worked for the bonding task, I also noticed that the dried leftover material (I mixed it in the underside of the bottle cap) was hard enough to sand, but not rock hard after 24 hours. When the right opportunity presented itself, I mixed up a little bit and applied some to a resin surface that needed a little touchup. It worked exactly as intended. It is now in the arsenal of filling and sanding solutions and is a good choice for resin.
