chemical warfare? <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Start with rubbing compound. Maybe you need not to paint or gelcoat anymore, because the surface is again shiny.
If you don't get rid of the dirt, sandpaper removes nearly everything, but you have to polish or paint after.
In case of tar, pitch, crude oil... chemical warfare.

Cheers,

Klaus