Oh, boy, where to start..... There are several different types of fillers, each with it's own purpose. I'm no using the term "bondo" as it is a brand name. The gray/red is the common stuff that is used as a base filler. It cad be primed and painted over, but it is somewhat porus. There are also what they call "spot/glazing" putty. That is most likely the white stuff that you are seeing. It usually somes in a squeeze tube, and is either white or orange/red. There are also blue and green varieties as well, depending on maker. There are also finishing/glazing putties that requir a hardener like traditional filler, they are susually yellow/gold or white in color, depending on the hardner color.
Generally, you will use the regular filler to do the bulk part of your repair, then the glazing putty to get the smooth finish and to seal the base filler. The stuff in the tube is best left for small repairs, they dry like paint (evaporation of solvent) and have to be left to dry for several houres before sanding. The other types can usually be sanded in 15 minuits or so.
You'll want to get quality fillers, go to the flaps or auto paint store to get the best fillers. Personally, I prefer Dynatron's professional products, Dyna-Lite for base work and Putty-Cote for finishing. Auto-Zone carries them. Bondo brand is also good, they are a division of Dynatron. Evercoat has quality products as well, I personally have not used them though. You'll want to get some rattle can sandable auto primer to spray a guide coat to help find high and low spots when you sand. I prefer the dark gray myself as it's easier to see.
