Bondo. Small tube; never had it dry up. Lasts a long time. Good on metal (surprise!) too. Easily sand-able, you betcha.
But - I have noted (and repaired) some that I had used to fill some spots in an epoxy (JB Weld) patch. After a couple of years the Bondo turned to dust. I guess some Bondo ingredients and some epoxy ingredients react badly.