I should have made a picture of the effect
It just became a very rough texture compared to the smooth rest, but not all over though (strangly enough). So I was almost thinking it was something coming from the can itself...
Anyway, the spray filler stated that it would double as a primer, so I didn't add additional primer to it. However, the filler and spray can were of a different brand...so I'll redo most, trying to get everything to match.
And yeah, the 600grit sanding in between was of course also done
Did sand the entire thing back down with 180 though, but used 600 in between layers.
But thanks for the tips! Good to see a process that should work...now just to replicate it