I just sanded mine to scuff the shine off, and used a high quality rubberized undercoat. It has held up very well over the years, and still looks great. Really quiets things down too - especially the "tinging" sound that sand and pebbles make on the bare aluminum.
I was going to use the extra spray-on bed liner that I used on the body but never did.
Did the inside fire wall with Damplifier and Heat Wave. very happy with results. http://www.secondskinaudio.com/index.php
I planned to do the same as convincor. I sprayed the underbody with commercial bed liner. I have enough extra to do the wheel wells of the finished car. I'll just mask off the suspension and spray what is left.
David, I think it was the heavy duty 3M stuff in a big rattlecan that is carried by Napa auto parts. It's really nice stuff that hardens after awhile so that it doesn't feel sticky to the touch like some of the undercoats do.
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