| Diesel1- Using the foamgun the way *I* (and a few others) use it is kinda involved, more complicated than you might think. I hate to say "do a search" but it's more than I can key in again right now.
*VERY* abbreviated version: No, I wouldn't foam the whole car. Rinse entire car, then work a panel at a time. Foam for presoak, rinse off. Dunk mitt in wash solution and spray foam at point ahead of/where mitt contacts paint. Rinse mitt often (at least twice per panel, more for big ones).
Best results on very dirty vehicles come (IMO) from incorporating a BHB into the process and the BHB might be the best way on *any* vehicle (jury's still out IMO). | 
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