The spot putty is just that, for small spots that need to be filled before final priming. It is more like a primer in paste form. It can be used over primer and sands real easy. It will easily get pushed through holes, not designed for filling large areas. I would use the bondo for filling holes and large gaps etc. The bondo will need to be used directly on the metal, not on top of paint or primer. You can also use modeling epoxy putty or squadren model filler, you can get these at a hobby shop.