Your best bet is to wash it with a decent household soap, and use a wax remover. Yes sanding will get rid of most diry, but you also dont want dirt/sand/bird **** scratching the surface you're trying to get totally smooth. Plus, all that stuff will chew up your sand paper/block.
When you think you're done sanding, sand again. 90% of painting is the prepwork. Do a few light coats of color, then do a light sand, followed by a few medium coats, with sanding in-between, then one 'wet' coat, with a final sanding.
Repeat that with the clear coat, and you'll have a damn fine looking spoiler.
