Have been playing for the first time with with BONDO to fill in some siding
cracks.

Boy, the stuff sure sets-up quickly; a matter of a few minutes.

Is there any way to lengthen the set-up time, without degrading any of
its other properties ?


Just use less hardener. I like to make a round circle of bondo and then
make a smiley face on it with the hardener. That seems to be about
right for me. It does harden to a non-workable state pretty fast, no
matter what you do, so you have to be ready to go with it.

If you are using Bondo on siding, make sure to paint the heck of it.
Bondo will absorb water if left uncoated. I'm not sure what the result
is on the side of a house but on a car it causes a mess.

There is fiberglass filler that you can get that is waterproof. It's
harder to sand and work with but you don't have to worry about failure
due to water.

In your case, this news is probably too late but something to keep in
mind next time.