Bob Geco and others,
The "flower pot" (registered trademark of Dan Caron) inside our W113 fuel tank makes the fuel tank a most interesting beast.
While I am sure there are exceptions, I'll offer a general rule of thumb: if your gas tank is rusting/corroding/self-destructing in any manner from the inside--enough for you to warrant considering some of these "renew" products, you might have enough sludge and what have you built up in and around the flower pot (which has plastic parts in it mind you...)that makes it difficult if not impossible to renew with any of these products. I've heard of some success--but I tell you, the local gastankrenu franchise was familiar with this flower pot and would NOT do it on my car.
JA17 Joe Alexander has posted pictures of this flowerpot before. If you think of some kind of coating sloshing around the inside of the tank, and you think of how this flower pot works with the fuel delivery and return, you should conclude it's probably not a good idea.
I had a bad gas tank and struggled for 3 years trying to find some OTHER problem. When I think of all the time, energy and money I wasted trying to circumvent buying a new gas tank, I cringe. At the time this was happening nobody suggested the fuel tank except one mechanic.
If your tank is original and you are having problems, replace it. Keep your tank full particularly when in storage.
If you want to inspect the inside of your tank, consider something like this:
http://www.ridgid.com/Tools/SeeSnake-microIt's a $200 tool--a solution in search of a problem.
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America