Rust V. Undercoating - PeachParts Mercedes-Benz Forum

joshhol · #1 11-15-2003, 06:25 PM

Rather than search the tomes, I thought I'd glance the subject briefly here.
It seems underneath the undercoating on my 300D there is rust. A shop was doing a minor repair underneath and reported that as they peeled the coating away, they just kept encoutering more rust.
It's a California car and I guess I could avoid driving in the rain as much as possible. What can I do? Have someone strip the underbody, apply copious amounts of POR15 and then recoat?
They were saying to do it right would require sandblasting and even then, if you even touch the metal, oils in your finger will get sealed in there and the rust will come back. They seemed to imply that no matter what, the rust will always come back.
What to do. . .

leathermang · #2 11-15-2003, 06:40 PM

What is the problem ? You have all the right information... just DON't touch it with your hands... that goes for ANY paint job... sand blasting is the ideal surface ( vacuum it immediately ) and apply whatever you are going to apply RIGHT THEN.... surface metal sandblasted can show oxidation within minutes... so many times since on a bad/long job you can't get the whole thing done before the first worked on part starts showing oxidation.. is to sandblast, vacuum, and then at the last minute when you have whatever paint you are going to use ready in the can... put Metalprep on it... this is phosphoric acid ..... and will get you back to NO rust under the coating you are applying...follow the directions closely on anything you use... the people who made the stuff have huge R and D departments.. and want their products to give you the best possible results.... POR15 is particularly critical as far as following the directions...but should last longer than your lifetime if done correctly...

JimSmith · #3 11-15-2003, 06:40 PM

joshol,

Your original suggested solution will work just fine. The problem with the plasticized undercoating is, as you have discovered, it ages, loses its flexibility, and cracks. When that happens it is worse than no undercoating. It traps water against the structural steel, and, along with the aging undercoating "exudates" (stuff the undercoating exudes as it ages and changes chemistry), it accelerates corrosion. POR 15 comes with cleaner and prep solutions that will remove nearly any traces of the stuff, after it has been chiseled or ground off, or sandblasted, and then chemically bonds to the steel. I am treating the jacking holes on my 1988 300E as they are showing signs of damage right around the holes. I hope I don't go an expedition across the whole bottom of the car. It was in Utah for a while so I don't think it will be that bad.

Good luck, and I hope this helps.

joshhol · #4 11-15-2003, 07:35 PM

You guys are right. I think the guy was giving me the "this is an old car you don't want to spend that kind of money let it die" speech. Might as well be speaking a foreign language.
Well I'll add that to my list of things to do, which keeps growing, as I have not had much motivation lately.
I've come across posts about undercoating removal, and understand it to be dreaded.
Jim that chemical change in the aging undercoating sounds particularly vile.
Probably course of action would be, grind strip and blast away undercoating, apply MetalPrep and POR15, then reapply undercoating. That should hold it for a while.
If the rust has migrated to the inside of the floorpan, then that will suck.
Thanks for your advice!

Cressida · #5 11-15-2003, 08:14 PM

The spray on "rubberized" undercoating that I've used seems to flow on pretty smooth and cover well. (It cost a few dollars more per can than the asphalt based updercoating)

I'm just wondering if one is better off spraying the rubberized undercoating on the bottom of a used car or just leaving it alone. As was mentioned on this thread, some seem to believe the undercoating may eventually causes more rust than if left alone.

So far the undercoating that I've applied a few years ago seems to be holding up just fine. Is it just a matter of time or what other factors make the undercoating start to break down and allow moisture to get trapped and the rust starting? Bottom line, are you concluding one is better off not to even use the undercoating at all?

joshhol · #6 11-15-2003, 10:24 PM

JD
From what the more experienced members have said, it depends on the car.
My car is 27 years old. It's been in California almost it's whole life so it could be a lot worse.
If you find pieces of the undercoating that have cracked or fallen off, you might see some rust on the metal underneath. If you suspect trouble, the thing to do would be start peeling away and finding how much rust there really is.
Not having any undercoating at all would only be better in the case of old undercoating that has undergone the chemical change as a result of aging. And in that aging process, it has started to crack and pull away from the body allowing water to enter in between the sheet metal and the undercoat. This water would be trapped inside, unable to dry easily, creating more than ideal conditions for the rust to form. In this case it would be better not to have the undercoating. Otherwise, it does it's job.
I don't know if it's normal to add more coating on a car you buy without removing the old stuff first.
I'm going to avoid driving in the rain until I deal with this.

Kestas · #7 11-16-2003, 12:13 AM

Joshhol, rustproofing (or any steel coating) that gets hard, brittle, and delaminates invites poultice (or crevice) corrosion, which is the worst type of corrosion. If you're looking for a one-time rustproofing application that lasts forever -- it doesn't exist. Effective rustproofing requires diligence and maintenance during the time you own the car. I've lived in the Rust Belt my whole life where corrosion is about as bad as it gets anywhere. Only the Maritime Provinces of Canada are worse. Over the years I've achieved some pretty good success with keeping my cars from rotting out.

The following link contains more thoughts from myself and others on rustproofing:
Rustproofing

joshhol · #8 11-16-2003, 05:21 PM

Thanks for the link Kestas.
I'll look into that counteractrust.com product.
I could combine that with POR, and a wax product.
Can the Texaco product be applied over POR15?

Kestas · #9 11-16-2003, 10:30 PM

Since that thread was written, the product, counteract, has been viewed more skeptically by others and myself. The concensus is growing that this is another "snake oil" product. I've studied corrosion when I was in school. The way Counteract works just doesn't jive with what I know about corrosion.

I'm not familiar with POR15. What is it?...and how does it work?

Another product I've used in my early days is roofing tar. I've never had a car done with tar long enough to see its long term effectiveness. My gut feeling tells me it may harden with time and get brittle. I use it on surfaces that are near the waxed surface. I played with roofing tar after reading an article back in 1974 Detroit News about a guy who bought a new 48 something, immediately put it on a rack and tarred the entire underbody. He drove it 20 years with no rust until he retired it as a summer car.

Being a young kid working on rusty cars I was quite moved by this story. This was quite an accomplishment since back then cars would perforate from rust after only 3 to 5 years. After spending untold hours fixing rust on cars (mine and family), I vowed never to have to deal with that again. Prevention is so much easier than fixing rusty holes.

joshhol · #10 11-16-2003, 10:57 PM

www.por15.com

It's not paint. Paint allows moisture through, so I'm told.
POR is cured using the moisture in the air. It kind of looks like a powder coat when it dries. It is flexible.
Highly regarded among most people I know in this forum.

Thats too bad about the counteract rust product. Wonder how Rogue is doing with it.

rwthomas1 · #11 11-16-2003, 11:22 PM

I am about to try Waxoyl rustpreventative/rustproofing. I bought enough to do my work truck. Its an oily/waxy substance that apparently never really dries so it continues to creep into cracks, etc. Popular with the Brit car crowd. Apparently it has to be maintained with a fresh application every year or so. Still cheaper and easier than fixing rust. RT

leathermang · #12 11-17-2003, 01:26 PM

http://www.metalwebnews.com/howto/rust/rust.html

Cressida · #13 11-17-2003, 04:03 PM

Quote:

Originally posted by Kestas
Another product I've used in my early days is roofing tar. I've never had a car done with tar long enough to see its long term effectiveness. My gut feeling tells me it may harden with time and get brittle. I use it on surfaces that are near the waxed surface. I played with roofing tar after reading an article back in 1974 Detroit News about a guy who bought a new 48 something, immediately put it on a rack and tarred the entire underbody. He drove it 20 years with no rust until he retired it as a summer car.

If considering roofing tar go for the higher priced rubberized products as they tend to spread better IMO.

The roofing tar that I applied about 5 years ago looks the same so I believe it is holding up just fine. The problem I've run into with tar is how to get it on the car. It can make a big mess. It is hard to spread out. Cannot really get it into tight spaces.

I just trowled it on the places I could get to easily. The places I could get to I was happy with the results. Use some old or junk putty knife or tools to apply as you probably will want to throw them away afterwards.

joshhol · #14 11-17-2003, 05:37 PM

Thanks for the link leathermang.

joshhol · #15 12-07-2003, 06:16 PM

Funny thing, I was under the car the other day, and the undercoating looked fine. It wasn't cracked or peeling at all and looked very sound.
There was a spot here and there where it had peeled away, but even under that the metal was not bare. It still had a black primer on it. I have some doubts about this guys claim.

So, I'm not going to worry too much about it for now. I will have to dig deeper though to make sure.