The problem with Bondo is that it's not a structural material. It's A, brittle B , porous and C, relatively soft. Epoxy will work, it's just slow and messy. If the resin you're talking about is polyester resin (ie: fiberglas), same comment as epoxy only slower and messier plus stinks to high heaven (we're talking respirators here, toxic) The way you're going about your model is fine. Here's something I do: I use water thin superglue as a primer for Bondo. Just paint over styrene and the bondo will stick to that. It also helps if you sand the styrene before applying Bondo. Sanding gives Bondo something to grab on to. Finally I brush Water thin super glue over to seal and give it a nice candy shell. Or more correctly, acrylic shell.