when you buy your "bondo" that is a trade name but I assume you use it
generically for other brands of body fillers.
Try asking for the polyester filler instead of the old style bondo. it
is much easier to sand and doesn't have that sticky problem.

If you already have bondo and want to use it, try not to add more
hardener than you need, and when it is still a bit soft , rough it out
with a serated plane , this will remove that top surface.

I am not sure if the newer type of filler is actually polyester or if
I made that up, ( maybe it was bondo that was polyester? ) but my
point is that the newer type of fillers are much easier to sand. also
they sell different weights. the "light weight " one is the easiest to
sand and probably less durable , like on a corner where it will get
knocked.