re: Anyone with body shop experience? Need help painting my son's go-cart.

What Shexter says is correct, but if you've let it sit for 2 days you could get some gray scotch brite hand pads and sand it with them wet. It will be a lot quicker and easier than wet sanding it with paper again. All you need to do is rough up the surface so that the paint will have something to stick to.

A good trick to go by is when you're using the scotch brite wet, water will bead up and not run off of spots that aren't sanded good enough. Once you get it sanded good, the water will cover the metal and stay put for some time until it either dries or runs off.