Thanks Jim, I ended up doing it again (4th batch of filler) and using 320/400 to sand back as gently as I could. Most of it seems to have stayed in the wood this time but there's still areas I'm not happy with.
Then do it again. They generally say 2-3 applications are needed (though I'm not sure if that's a ploy to sell more grain filler, like the 'repeat' instruction that got added to shampoo bottles).
I've just been spraying a black burst on my ES-3. I could have done yours at the same time if you'd popped over with it. Still got the black in the spray gun if you hurry...
Did my first gentle sand back after what must have been half-a-dozen layers of varnish.
All bloody came off around the edges. Complete balls up. Managed to do a bit of touching up but there are some very ugly bits on the front edges now.
Beginning to wonder is some kind of spray poly/clear coat might have to be the way to go.
I am not really enjoying this build at the moment.