Spray or brush on material for inside sheet metal?

These damping sheets from PE will accomplish what you are looking for in a very economical manner. They are more effective than using the "spray on" rubberized damping compound that you can find in some car audio and / or automotive supply houses. Sean
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Buy some Dynamat Extreme from your local car audio shop. Apply it to the cover, and anything else inside that seems flimsy or rattles. You can also use rope caulk (window putty) to damp any small components that rattle or resonate.

Just out of curiosity, why are you wanting to dampen the inside of the chasis instead of doing it from the outside? Have you tried mass loading the the top with something like a Bright Star Audio Little Rock? I use a Bright Star Little Rock 2 on top of my CDP, it weighs about 27-28 pounds. I ran across a post on time where a guy took a cigar box and filled it with pennies and used that to mass load his CDP. I was just wondering if you have tried anything like this yet. If you want to do the inside, check out a company called Cascade Engineering, I've seen some of their products used in cars. They might have something that will do what you want.

BTW, if you want to try a really great power cable, try one by Virtual Image. You can read some of my posts about V.I.'s cables here on Audiogon.