1) On bare metal, what grit should it be sanded with? The metal has been stripped with paint stripper and strip disks, so it has visible scratches all over it. I'm thinking I need to sand it with something finer to lessen the scratches.
-Bare metal I would sand up to 400 then shoot it with a primer. I would shoot a couple coats of primer then sand the primer smooth and try not to sand through all the primer, then paint.
-If the visible scratches are not blending well with the 400 grit then try 240 then 320 finishing with 400.
2) I've repaired several spots with new sheet metal and body filler, and those are sanded smooth up to 400. Should I go back and rough those up with something more coarse before priming?
-The 400 grit paper will be fine with the body filler. A good idea before priming would be to go over the surface with a red scuff pad made by 3m. It will help adhere the primer to the body filler and any other areas not sanded thorough enough.
2) My rollbar is in good shape and doesn't need to be stripped to the metal. So I need to sand it just a mininmal amount. What grits of sandpaper should I use before priming?
-If you want to just repaint I would go with a 800 grit or 1000 grit sand paper. But if you have chips that you are hopping to cover then i would sand them smooth to a blend with the rest of the paint with some 320 grit. Then prime those areas, followed by some 400 grit sanding to smooth the primer, finally paint away.
If you have any other questions feel free to PM me.
bare metal- shouldnt be sanded with anything finer than 180 grit, i actually prefer 120 grit. 400 grit is too fine, even if an epoxy primer is used......ask me how i know......i did a nose on a grumman bread truck, sanded the bare aluminum with 400 grit, epoxied(as a sealer) and sprayed single stage on it. guess what was back in the shop 2 weeks later with almost all the paint blown off from stone chips....the grumman van.
second off; when you do a a spot of body filler, your rough out work is up to 36/40 grit paper, second coat of b/w is 80 grit, and final putty/glaze is done with up to 180. you use 400/320 for your feather edge to spray the primer into. usually all body shops have only a few grits of paper they use. it cut alot of cost down.....rule of thumb on grit paper to use when ya start a project........36/40, 80, 180, 320/400, 800wet, and 2000wet. they are really all you need to do any project.
your roll bar; with nicks and scratches sand with 180, without nicks and scratches 320/400. you will do more damage trying to feather small spots with 400 then with 180, plus it'll take ya twice as long.