When my Son uses the Rust Stop type of product, after he has removed the loose rust with sandpaper... how long does he have to wait after spraying it on, for it dry and then after it dries, does he have to sand it before beginning with the body filler work? THANKS:confused:

I would spray it on as soon as it's clean and dry. I would not sand it unless it "ran".

That rust stop isn't like ordinary primer. I believe it wants a 24hr drying time. I thing it actually reacts with rust to make it inert.

24Hrs unless the directions say otherwise.