Oops! I've done a silly thing. I sprayed on some spray filler without mixing in the hardner. It has "set" to something like putty. I'm going to scrape it off and do it again properly. Do I need to get every trace of it off or will the hardner in the next layer cure any remaining traces in pin holes etc?

I have done this before... Just make the next layer a bit stronger with more hardner... get it applied quickly so it doesnt set, and it should then cure the rest of the filler

If i was you i'd take it all off. You wouldn't build a house on dodgy foundations.
It's pain, but you might as well get it right now