Why would non rusted (floor pan) sheet metal sprayed with undercoating be prone to rust? Is the metal in these cars just that bad? I'm assuming the 33 yr old undercoating was crap, but i would think the newer stuff would lock out moisture? It would kind of seem pointless to sell undercoating if it didn't protect the metal no? That's what it's made for.

Not implying anything, just asking.

How do newer cars avoid this problem? There undercoated no?

  Jayru said:

Why would non rusted (floor pan) sheet metal sprayed with undercoating be prone to rust? Is the metal in these cars just that bad? I'm assuming the 33 yr old undercoating was crap, but i would think the newer stuff would lock out moisture? It would kind of seem pointless to sell undercoating if it didn't protect the metal no? That's what it's made for.

Not implying anything, just asking.

How do newer cars avoid this problem? There undercoated no?

First, undercoating is not a 'Rust Proofer".

Second, Undercoating get's damaged by the elements. Rocks, Road Salt, Chemicals, etc which allows moisture etc to come in direct contact with the metal and traps it there.

Third, the metal on these cars is thinner than modern cars, that's why these cars are light. Also the metal was not well treated for corrosion resistance

If I may intrude....

Carl is spot on. He should be--he's discussed this loads of times.

Undercoating is mainly there to deaden road noise and provide some thermal insulation. Oh, yeah--since it is porous it also does a heck of a job keeping moisture up against the poorly-prepped sheet metal of the floorpans. So, the "tip of the iceberg" is very true to form on these cars.

Take off the tar crap, I suggest heat gun and paint scraper, but it sounds like dry ice also works from other folk's impressions. I then took a small skinny screwdriver and jabbed it into the floor pan, "mining" for rust. I found it along the front firewall, drain plugs, seat mounts and along the rearward weld line. Not much, but the pock-marked looking pinhole type problems. All of this was covered by the tar insulation, giving the intial impression of being "rust free." "Rust free" and "Z" rarely go in the same sentence!

Grind away. Wear a mask. When everything around you turns dusty-orange, you may have had enough fun with the grinder. Vacuum, rinse, then scrub it down with Marine Clean to get the last bits of grease and grime off. Rinse again and allow to dry. Since the bulk of my pans were pretty solid, I used Metal Ready to get the good metal prepped. (POR Inc. states you don't need to use MR if only applying to rusted areas.) Rinse again. Apply the POR in multiple thin coats according to the directions. Cover the floor pans entirely, you will thank yourself later.

Work in a well-vented area. Wear a mask. Wear gloves. All those kind of things.

Hope this helps.

X

In addition to the previous advice I found that a wet/dry vac is very useful when working on the inside. Several of the POR15 steps require rinsing with water and a wet /dry vacuum will get most of it out in a hurry. There's also a lot of other debris that comes out with it. I know that this thread was started talking about the underside of the floor pans but to do it right you really have to treat it on both sides. If there's rust at all under the undercoating there's going to be rust under the tar mat.