What grit sandpaper is the best to get all the paint off my bike?

why are you worried about your bike being repainted? maybe you should worry about school that is coming up in a couple of days? Go drink some milk and sleep early for preschool little boy!

If you're repainting the bike, then just use a 36 or 50 grit paper and leave it at that....it'll give the primer something to stick to. If you're going to strip it and clearcoat it, you can start out coarse and work to a finer grit or just use a finer grit along with a lot more elbow grease and sheets of paper.

A great tip is to buy sanding belts and snip them into pieces rather than using paper-backed stuff. It'll last MUCH longer and you can flex it into the corners of the frame joints without it disintegrating on you. Also, you might find it faster and easier to use a metal scraper at first to remove the bulk of the paint, then finish up with sanding.

Never use lower than 100 grit to remove paint on a bike. You'll scratch the metal badly and paint will not fill in the scratches.

You should also avoid garnet paper (the brown stuff) and any weird color stuff (blue, green, orange). These have coatings on them that might prevent paint from sticking. Instead, use aluminum oxide paper that is either black or white.

Start out with 80 or 100 grit and work down to a fine 150 or 200. I like to use wet sandpaper and fold the sheets to 1/4 size and tear. Using belt sandpaper is expensive and is not water proof. Dip the sand paper into the water often to wash out the paint from the paper.

Start with a low grit to get all the paint off, than switch to a higher grit to smooth it out. Make sure to rinse off your bike between sandings.

Sandpaper? I'd be using paint remover if I wanted all of the paint off my bike. You're giving yourself a lot of work to try and sandpaper the paint off.