avoid bondo brand. it will function, but is a pain in the ass to work with. use a real industry standard brand like Evercoat or USC. I use
Evercoat Rage Extreme but anything from Evercoat is decent. think about getting something thinner like glazing putty if you're doing light skimcoat filling only.
USC Icing is my fav there.
the "real" brands cost more, but IMO you should buy good materials so your repair turns out as good as it can.