if the a/c was a dealer installation then the dealer would have to had to lift the mats to install the drain lines anyway. the metal would still be pristine and the dealer would have had access to the tar type material as you call it
ron

http://s908.beta.photobucket.com/user/padresag/library/turned%20works
http://s908.beta.photobucket.com/user/padresag/library/furniture
and a little history of where I come from
http://forum.woodenboat.com/showthread.php?156542-old-boatshop-in-Steveston