Tab, the metal wasn't ready for filler, that is why it was flimsy. I work with this stuff every day. It is seldom I work on anything older than five years old and mostly import.
You certainly don't need spot putty to work on them. If your polyester putty is kicking too fast, back off on the hardener. Polyester is thermosetting so super accurate mixing isn't as critical as say a urethane or epoxy.
Something like Metal Glaze or Glaze coat sand just as easy as the spot putty. This is digging WAY back in my memory banks of course, I have not sanded on stroke on spot putty in 20+ years.
I had a flimsy panel the other day and pulled out my shrinking disc just to firm it up. I still used filler, but in a quick minute the panel was firm for the filler.