I know body filler is very porous.

Does it absorve water when it is hard??

Will it absorbe prep-solve the same way it will water???

Humidity is a kller and bondo will absorb it,just as laquer primer does(not that alot of people use that stuff anymore)If your work is clean and you dont leave the metal open for extended periods you shouldnt have to use a solvent prep.If you do,You dont want the towel "wet" anyway,just damp.Ring it out before use and come in right behind it with a dry one.
Then acid primer-then sealer or primer to get it covered up. I also DO NOT recommend wet sanding sealer or primer.That puts humidity on the surface and if the primer or sealer are thin youre exposing metal to water= rust.
Wet sanding is for final paint prep before buffing to that miraculous show car shine