I'm almost up to painting the underside of my project.

K&H have a product called kahguard, it is available in black or white and used with a shutz gun which is actually very much like a spray gun but able to spread the thicker body deadener material all in one go.

I'd advise against using deadener and painting over it. Only due to the fact your paint will eventually be stripped off from road use.

As for cleaning the underside I'm not yet an expert.... But every car can be different!

I for one would go see a autobarn store and ask about kahguard in there paint department