Body filler has a perfect time to be 'roughed in'.
- Wait too long: the filler takes too long to sand, and because it's too difficult to sand, it's more difficult to get it straight.
- Start sanding too soon: the filler will just ball up and not sand. Also, there is too little adhesion to the panel when he filler is too wet, so it is very possible to peel it off the panel.
- The Perfect time: Just after the point when the filler balls up in the sand paper. It will still be tacky, or 'green', but it is much easier to block down. It should look like larger chunks of dust, not rolled out balls of wet filler.
- The Grit: Rough filler in with 36 grit. The idea at this point is to only get the general shape of the filler correct. There should be enough filler left on the panel to block sand out the 36 grit with 80 grit. 36 is very aggressive! Don't sand too far with it. Also, that aggressive of sand paper helps get the body filler even straighter, because it cuts so sharp.
Above: Sanded too soon
Above: Sanded at just the right time. It's hard to see, but the 'dust'needs to look like that