the auto body filler you are using (feather fill, mico-lite, est) two part mix is good stuff but learn how it works.
get some scrap glass(fuzz cutout,any hardened glass) and drill in a small pee like hole then sand over the hole with 36# then farther out sand with 80#
then with 180# and finally the edge with 320# now fill over it with your putty. now sand it down to 150# just the same as you would if you wanted to prime it, don't waist your time priming it just yet, but rather insert it it to the oven at the lowest setting for 4-8 hours (same as parking the plane out it the sun.
remove from oven and look at what you see, a hole with scratches all around it. re-sand and prime, it will now work just fine in the sun.
putty works a lot like are epoxy as it needs to post cure.