That is what I meant if it doesn't sound like it. Even thought of a way I could make a water-trap filter if we used the door to the outside as the intake that would probably work better then an off the shelf filter.
Wet your floor and put the fans in front of the door while its cracked and all dirt will stick to wet floor allowing clean air to move across it. Will this stop 100% of the dirt, Hell no but if its that bad you need to put it in a real booth or build you garage into a better filtration system.
I do like the idea with the bucket of water to catch paint overspray, I may have to steal that.