If you're leveling a whole car in prep for paint, and the surface is dicey, I'd try something like Slick Sand 2K primer. It's made by Evercoat and is very thick, so it needs a big tip to spray, like a 2.0. But it sands quickly and levels nicely. It fills some pretty healthy inperfections, but needs to be covered with primer and sealer before paint. It sure does make a wavy surface straight in a hurry though.
Can it be left on without primer/sealer if the car is garaged? What kind of primer is needed after using this stuff? The car is pretty straight except for one spot on the door and the tops of the fenders
Personally, I wouldn't. It's very absorbent and not waterproof so it absorbs water from the air just like plain lacquer primer. The good news is that it sands easily and quickly so you could shoot it one day, block it the next and then shoot some epoxy over it to seal it and it would be good for as long as you wanted to wait. I also know a local painter that lets cars shot with it sit for 30 days before sanding it, but that's in a heated shop.
so what kind of primer should I use over this stuff? I am blocking the entire car with 80grit, then using the primer filler you mentioned, then blocking with 80 up to 220, then shooting the primer over that. I am assuming that is the way to go.
I think that's a good strategy, as for primer to use over that I'd use an epoxy. If you already know what brand of paint you're using (PPG, DuPont, Omni, HoK, etc) then try to use that particular brand of epoxy primer. The nice thing about epoxy is that you can let the car sit until you're ready to work on it with the fear of it rusting through the primer. I saw a '67 fastback in a bodyshop a few months ago and they stripped it to bare metal, did some filler work and then let the car sit for a few weeks. When I saw it, the body was coated with surface rust and the rust was even under the filler. That means they get to either re-do the whole thing, or try to hide it from the owner by simply shooting epoxy over it, which is a 100% gauranteed failure. But if you shoot epoxy over filler or bare steel or a 2K primer like you're considering as soon as possible, you'll be fine.