I have a model which is plank on frame construction, the model was left in a warm room for many years and the planks have shrunk.

If you are painting it, it's probably not going to matter at all. Often car body fillers are self-priming, so they will be ready for paint straight over the top. They are generally also easier to sand than fibreglass, especially if you are using epoxy resin. You could just use a fibreglass reinforced filler, though I'm assuming there is a reason you want to do both filler and glass? 

Thanks for your reply.