You can wet sand plastic body filler ("Bondo" is a trade name like "Chevrolet" or "Nike") but you should carefully heat the filler with a blow torch before priming. Just take a blow torch and quickly wave it across the filler and you will see the moisture evaporating right before your eyes. When all of the moisture has completely evaporated, prime it immediately. If you leave the moisture in the filler and prime over it the filler will crack.
The torch is not a bad idea. I would definately say dont coat it without letting it dry good. You can see when the moisture is gone. It just takes a couple of minutes. It will be darker yellow, and then fade in color from the outer edge in as it drys. Usually takes about 5 minutes after you wipe it off with a dry clothe.
Be careful with the torch around filler, if the filler isn't fully cured and still releasing gases it could light up your life. You could also potetially melt the filler if you don't move fast enough or are holding the torch to close. This will also shrink the filler a little so if you make the filler work perfect and do this method don't be suprised if you have to go back and do a little more work. Its not definate, but any of these can happen.
The reason I want to do it is to use a little finer paped to get the edges blended in better. I am useing 220 and it keep clogging. also the rest of the fender has primer that I need to sand down and the paper keep clogging on it also. It drying it is the only problem Iam not worried about it. I can let it dry enough. I just didnt know if it would hurt the bonding or anything.
Get the bondo close w/ 100grit paper then pile on a few heavy coats of hi-build, sandable primer and then you can dry sand to perfection w/o the clogging trouble. If you are unhappy w/ the coarse sandpaper scratches, squeegee on a thin filler coat of two part EverCoat polyester spot filler B4 priming. It seems to dry sand a little easier than bulk filler. Main thing is to do your final blending and finishing on the sandable filler/primer.
One thing to keep in mind is when bondo get's wet it absorbs water like a sponge. What's behind the wet bondo you say? Bare metal! you get it wet....it rusts. This is the number one reason not to simply prime your car and drive around without painting. I would dry sand the bondo.
The only reason for wet sanding is to hold down dust. It really has nothing to do with a smoother serface although you do have to use a finer paper when dry sanding to get the same smoothness you get when wet sanding.
I did my old motorcycle a couple of years ago and wet sanded the spots where I filled in the dents with bondo. Looked very nice. I wet sanded to keep sand paper from clogging. After painting it and airbrushed on a pic I got one bubble pop up the size of a quarter about an 1/8 th of an inch high. I figured it had something to do with the bondo. Now I know for sure. Sanded real well but didn't know it would bubble like that. Learn something new everyday. Even some useful things.
I wouldn't wetsand fillers for the same reason and sometimes use a blowtorch to remove bondo, it just disintegrates into powder, very easy to remove. If you ever get your filler wet, dry it with a heat gun or hairdryer, not too close, take your time. If you have some very deep areas, let them sit awhile & do them again, but personally I myself wouldn't trust it if it got wet, I'ld rip it out & do it again, learned the hard way, good paint's expensive.
Willy 36 has got you going in the right direction.
Don`t get the filler wet. If you do, Use a heat gun.If you want straight sheetmetal stay away from a torch.My self I use 120 gold 3m dry paper in a roll.
When the filler starts showing through the primer, you should be done, If the area needs more blocking, shoot some more primer,let dry , start blocking again,keep blocking untill lows and highs are gone and no filler shows. Exp. is your best teach. Good Luck