There is a product to change rust to a primer type surface. I did this repair to my 56 Nomad wagon back in the 70's. Really poured the stuff on after removing all loose surface rust. Then did it again 2 more times. Back side covered with a fiberglass/roofing material.....allowed to harden and then painted with a flexible epoxy paint on the backside at the last of the repair sequence (as best I could...not being the best contortionist in the world). Then, on the outside, stepped the metal down and welded in home made pannels of galvanized metal. Then on outside, just as little bondo (ALL STEEL -type or brand name if I remember correctly) as possible. Again, an epoxy primer and paint. Has been 30 some odd years. Seems to be holding, but for how much longer????????

Regards

Run-em

theres no doubt if the metal is perforated it must be replaced.cut out the area carefully with a dremel.use the removed piece as a template to fabricate a new one.if you trace around the old piece with a pencil and cut out with the dremel the new piece should be a close fit.the best and most rust resistant welds are butt welds.avoid overlapping the patch as this creates moisture "traps".a hobby size wire feed welder is perfect for creating the small spot welds required.a series of numerous small spot welds will hold the patch in place securely and require little grinding.finish with a super thin coat of bondo,sand and paint accordingly.Sounds easy doesnt it?a little practice will make you an expert.

After seeing some amazing rust on my 300SD I have come to the conclusion to avoid it all together. So any MB I buy next that I plan on keeping for awhile I will buy from a rust free area of the country. Then I will just not drive them in the winter.

Rust is like a cancer if you see a little spot that is only part of it.

Brian your mistake is using Bondo, you need to treat the rust with some sort of rust converter. Then use a filler that doesn't absorb water like Dura Glass.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

If the Bondo is primed properly and a finish coat is added properly, how does the Bondo absorb water?

I think the constant bashing of Bondo might be erroneous. Water can't get beneath the paint and above the steel panel. The rust must be returning from within the steel. This is where a proper converter might be of assistance, if it works.

Sure water can get below paint in the marine world this is quite common.

Bondo is made out of a type of epoxy that absorbes water, people who fair their keels and outdrives with it get nasty surprises in the fall.

Dura Glass is a body filler made for cars with fiberglass body's. But it doesn't abosorb water and can be used underwater.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

How does it get below the paint on an automobile, unless the topcoat(s) are compromised?

I'm sure it gets through paint isn't water proof like it appears.

Actually I know of a boat that had Petit Easypoxy applied to dry lockers a year ago. The owner placed a little wet gear in their but nothing that wet. Guess what under neath that stuff the paint is lifting and is wet.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

You perpetually refer to boats........wooden structures that are surrounded by water and gratefully absorb it.

I'm betting that the paint on an automobile is waterproof. If it was porous, every steel vehicle out there would have rust all over the body.

Too bad there does not seem to be a corrosion engineer on our site. Most short hair and long hair auto body filler is waterproof to the best of my knowledge. Normal bondo just absorbs water (hydroscopic I believe) if given the opportunity. Plus remember that laquer based primers let moisture through although I believe top coats or two part primer are waterproof. The iron rule though is not to bury rust. For if you do it will cause problems sooner or later. There is a specific surface rust rhat occurs on mercedes products that should respond to sand or media blast and metal prep before finishing area quite well. Also rust seems to be somewhat different depending on your geographic location. Same thing but just develops differently and to greater or lesser extent. I consulted a corrosion engineer years ago for a business. He said basically nothing really stops the oxidation process other than removal or true conversion to iron oxide with things like phosphoric acid. He was full of ideals on how to slow it up though. So unfortunatly the comparison of rust\cancer does bear some truth it seems.

The problem with bondo is that IF there is any moisture around, it WILL absorb it, then trap it next to any bare metal. Often the moisture is present before the paint is applied. Sometimes, filler (bondo) is used in places where moisture can enter it from the back side. I have a car that has parking lamps set into the tops of the front fender. The housing for them was originally leaded on to the fender (another story), but a restorer welded them and filled around the edges to smooth. The wire lead for the bulb came through the back of the fender into the housing--protected by a rubber grommet. It leaked. Moisture entered from the back and was absorbed by the filler. It turned the metal beneath it into rusty lace. From the top, the only visible evidence was bubbling at the edges.

Filler, used very sparingly, is OK--even necessary sometimes. Some shops use tons of it to cover all sorts of demons--including old rusty areas they did not want to take the time to fix.

Also: Paint is not an impermeable barrier. It breathes and moves with changes in temperature/humidity. When it gets old and has been exposed to years of UV, it cracks. Moisture creeps into amazingly remote places. Trapped there, the inevitable happens.

i realize this is not a restoration forum, but the principles apply to rust repair, generally.
Mark
93 300D 2.5
99 Crown Vic Police Interceptor
66 Mustang Coupe (restomod)
61 Rolls-Royce Silver Cloud II

and to emphasize, it is ok to use bondo on the surface of sound metal. as noted above any perforations will allow moisture to come in from behind.

for any areas with perf. you must use the fiberglass filler or metallic.

if i have rust to repair, i sand it and cut out as much as possible, rust bind it and sand again, and rust bind again, then prime and paint. as much of the rust must be removed as possible for a good job.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

http://ptcl.chem.ox.ac.uk/MSDS/glossary/hygroscopic.html

Newly sandblasted steel in a humid warm climate will show rust 10 minutes after being blasted. Before painting one must be sure the dust is off a piece...which usually means using the air nozzle... one must be sure that the air coming from the air source is as dry as possible... this needed to be the case for use with painting equipment and air tools anyway.... so care should be taken to really get it dry.
An extra cooling/drying tube arrangement is often worth the price... the little air filters coming off the air compressor are just the starting point.
Sandblasting is the ideal surface for the application of Bondo since this is a physical bond process compared to chemical. If you do it in a temp/humidity controlled space and apply it immediately.. then work it as soon as possible and apply primer you are in pretty good shape.
Primer is hyGroscopic. I laugh everytime I see cars driving around with primer on them.... the people did a great job of sanding.... but failed to read any books about Auto Painting...
I do think that properly applied Top Coats of automotive paint are water proof. Those long chain linking molecules are amazing....
The problem when painting a car is that primer ( which includes what is called ' spot putty ' .. paint with little carrier which is applied with a rubber squeegy ) absorbs THINNER, REDUCER, ( whatever you want to call the carrier liquid ) ... so IT EXPANDS with each primer coat sprayed ( and when the top coat is applied )....
so you really want to let the primer dry for weeks before applying the top coat.(also wait on top coat sanding) But that way you are leaving it exposed to the weather even inside unless your paint stall is temp/humidity controlled.
If you sand TOO EARLY then that expanded area which is slightly thicker than other areas will be LOW when it DOES get rid of the carrier.
I have tried and read about the rust converters... I do not trust them. Some may convert but still leave less than solid gripping surfances for the next coat. I don't know of many places on a car where butt welding is the best replacement choice. Too many places won't allow welding at all due to the insulation and bad stuff behind it or close enough that you can't really use big heat. Cutting out , flanging the sheetmetal with an air flanger, putting holes for both plug welds and rivets ( applied first ). In most intances the rivets done alone will be great ... and it means you can apply something like POR15 to the area after you have drilled your rivet holes...and put the thing together using the POR15 as a sealer between the old metal and your patch.
Before someone says something about rivets... remember that they put Airplanes together with them.... there are no doubt more kinds of rivets and more instructions about choosing and placing them than you are aware of.

yeah, i have used rivets many times. usually when done you can't see them, but if the car is just a driver for a new teen driver i have left them exposed on occasion as a neatly painted patch with matching paint is a lot better looking than a gaping rusty hole.

knowing that the odds of a new driver having a wreck are exceedingly high i never put too much work in the body til they had a year or two wreck free.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.