I have a 1999 C230 Kompressor Sport. I absolutely love this car. I have dumped near $3000 into repainting various parts to stop the rust.
The first picture is of my lower front door, this area has never been dealt with.

http://img394.imageshack.us/img394/2839/rust1eq7.th.jpg (http://img394.imageshack.us/my.php?image=rust1eq7.jpg)

These two pics show an area I have had "repaired". I spent $800 about 6 months ago having this repainted. Its the area where the bumper meets the rear fender. WHAT SHOULD I DO?????

I have used POR-15 on another part of the car and I think its holding up, but its in a very discreet area and I'm not sure I'm comforable using it on these.
http://img73.imageshack.us/img73/7972/rust2bs8.th.jpg (http://img73.imageshack.us/my.php?image=rust2bs8.jpg)
http://img394.imageshack.us/img394/1056/rust3tm8.th.jpg (http://img394.imageshack.us/my.php?image=rust3tm8.jpg)

Don't really have much good news as you seem to have a car with serious rust issues. An observation from personal experience is that Mercedes certainly had rust problems fairly early, on some cars from 1996 on. We saw some serious rust on some 03 and earlier S classes, where Mercedes even went so far as to replace panels under warranty. Looks like you may be facing a never ending battle as if the metal wasn't dealt with properly at the factory, you can never cure the problem,,, sorry. JMHO.

My niece's 1998 C230 had about 122,000 miles and it didnt have rust,but my brothers 1994 S500 started to rust a little bit at about 98,000 miles.Then again,my Toyota truck which has been garaged about 10 times all its life does not have a tiny spot of rust.Sometimes cars just act weird

I think that the purity of metal must have a lot to do with rust. In my job we design and fabricate equipment out of stainless steel. At times some of our stock has come from Korea (which seemed to be the worst) and other times it is American or some other country where standards are probably higher.

The Korean steel rusted and discolored. We had various problems with welds failing, etc. I believe this was due to impurities in the material.

It's hard to imagine MB using steel with impurities in it, but this sounds like the most likely cause of the rust and corrosion problems that have been mentioned here. If so, there really isn't much you can do about it.

OK, so I got back from my body shop and they said theu would redo that area no questions asked. They do wonderful work and are restoration specialists verus collision specialists. Although, they really dont specialize in rust. However, I think that the rust will probably come back again. Should I ask if I can por-15 that area and put the special primer on? Do you guys think that product would work in that area? I've only had it on one part of my car for about 3 months now, so I cant attest to its durability.

Also, I'm dropping the car of for Monday 8/21, so please let me know what you think.

Thanks a bunch....:)

Don't know if the POR 15 is intended for finish body panels as the primer has to be very thin to let the finish work appear correct. If the rusted areas are taken down to bare metal and good 2 part primer/surfacer applied, that should take care of the problem.