Sound Dampening Paint??
 

http://www.mascoat.com/dB.htm

Has anybody had any thoughts on using sound dampening paint inside the cockpit??

The weight is .4lbs/sq ft... how does that compare to other materials?

I know lots of guys who use this stuff in their car..

Works like a charm...

This noise damping has been discussed many times before here. If you wish to damp noise, it's just that you have to add weight -- the more the better. It's not so critical in which form it comes if you wish to do so. Of course later it becomes an issue that it takes too much space and therefore density (kg/m^3) has to be get better. So if you feel to add more weight to your plane, go for it!

Solution I offer for this noise is to get better headsets -- they cost only fraction of the whole project and will be on your ears whenever you fly. For example most of the propeller noise comes through the canopy -- are you planning to paint that as well? ;)

Use better headsets and save the weight
 

I totally agree. With my noise cancelling headset I do not think the RV is loud (or maybe I have gone deaf due to all that rock music I listen to). Other planes with the same engine that I have been in sound louder than my RV. My Cessna 140 seems to be louder as well, although I have not flown it in awhile now that the RV is flying.

I have been doing some interior work on the RV we bought and it is noticeably louder with the simply flooring the builder put in removed than before..