Rust-eater. Now there is a marketing phrase to laugh at! Making fresh steel out of Iron Oxide is a clever trick requiring a large factory. What all (I think I am pretty safe in saying this) 'rust eater' products do is chemically seal the rust after stabilising them. Usually a two stage process requiring an acid, followed by a latex based sealant. In the dim distant past when I used to restore Mini's and MGs (1960's versions) the metal of these cars were much thicker. As a result corrosion was often surface only and such chemicals worked well - for maybe 3-4 years.
Far better was to shot blast the rust away back to clean un-oxidised steel. Not always an option. Problems with resin fillers occur because the resin can trap moisture on the metal surface. I use an MB independent mechanic who worked for a big dealership for 15 years. He was taken to the MB factory and shown production run. The vehicles are/were hot dipped and agitated in a zinc solution. They also required an electric charge on the bodies as they went through the dip. I can not believe this is still done given the problems with W210s.
So, using a 'rust eater' merely stabilises the rust and covers it with latex to prevent most moisture getting at the surface and triggering further oxidisation. With paint over the top this will probably hold out for about 3-4 years in UK climate depending on how trapped/exposed the metal part is.
If the part is not seen, I used to rub old thick engine oil in the damage! Waxoyl is almost the same but with better gelling/stickability - and smell. Also without bits of shell bearing in too
My 1997 E300TD spent most of its life near the south coast and has the sand in the engine bay to prove it. It has had the recall reproofing of the sills (despite being out of VIN range for recall) but it does not have any of the rust I have read about on this forum. In fact it has a W124 trait (which I had two Harvest Gold 300TE estates at the same time once) of rear top wheel arch corrosion. As the paint is only bubbling at this point with no stone chips I could take it to the local MB dealer but I am not fussed. The car is over 7 years old with 115k.
One W210 I looked at had severe corrosion all over the boot and battery compartment floor. It seems that while quality control varied at MB between 1994-1999(?) this means some vehicles had poor paint treatment but some were OK. I seem to be lucky.
So back to rust. If Stealership refuse I would still use so-called 'rust-eaters' but I would try and use a two stage one. I would then put plenty of primer on - at least three coats, then at least two top coats, followed by laquer. Finally I would really wax the final surface if seen, of if hidden cover with a sticky oil like Waxoyl.
I did all this to a seriously modified Mini which I kept for 4-5 years at the same time a friend did his with just filler, a coat of primer, and top coat. Both vehicles had quite a bit of rust on them beforehand.
His Mini had bubbles in paint work around rust points within 18 Months. Not one on mine after over 4 years.
Mind you - the 2000 A170CDI I had for 18 Months was rusting on the roof in 2002! Too many warranty suspension rebuilds and my wife traded it for a 1994 Saab convertible - now that is a car that doesn't rust.
Great to see Forum being used to pressurise MB....