Eastwood Company has a "Gas Tank Sealer Kit" for around $35. The kit comes with a "meat wash", oxisolve (etcher), and sealer.
Checkout their site at, http://www.eastwoodco.com/cgi-bin/sgin0101.exe
Hope this is what you are looking for.
BTW, for anyone who has not used Eastwood products, they are considered one of the top restorer supply houses out there. They have many products that will restore and repair Mercedes parts.