Is undercoating good or bad ?

[olebluegmc]

I was planning on coating all my patch panels with 3m undercoating where it won't be getting welded, like my cab mounts, and rockers on the inside where it won't get welded. I Thought this would be good to prevent future rusting out of these troubled areas, however I was talking to a mechanic at work and he said undercoating causes more rust because the moisture can get trapped underneath and expedite rusting. Looking to see if anybody has had good or bad issues with undercoating ?

[hugger6933]

I think your guy has it wrong the undercoating prevents the moisture getting next to the metal. But with that being said if you put it on and weld close to it it can/will melt away from that area. If you leave a big portion untreated so not to get burned what would be the use in some if not all? My ideas I will submit is to use weld through primer then you want extend it out past the areas to be welded then undercoat the parts away from the heat. Or treat the entire panel with weld through. Any way you go is a better than what they had as in nothing.

[tommys72]

Just get an oil leak like I got, it'll undercoat everything for you! Haha

[jocko]

If you've got it rust free and are gonna park it inside - I'd skip the uc. If you're gonna drive it in salt, might consider it, otherwise no. Might want to look for another mechanic... JUST kidding. Actually, maybe he's onto something - several of these trucks I've seen that had uc, I noticed when I peeled it's dry, rotted, mess off that there was rust underneath. Hmm. Ok, give that man a raise. Bottom line, I wouldn't undercoat it anyway.

[agawam]

3M undercoat will cost a lot , its not cheap, what I use is non fibered roof tar mixed with a little gasoline, it is a real petroleum product ,the gasoline aides in drying, you can handle it in about a hour, dries like paint, spray out of a body shutz gun, a gallon at home depot is around 16 bucks an goes a real long way

[Wrenchbender Ret]

I think black satan paint looks a lot better. If the metal has surface rust, spray it with Rustoleum Rusty metal primer.

[olebluegmc]

I was mainly planning to do it on the inside of the cab mounts/ supports since I have access and am putting new ones. They seem to rust out on the bottom and sides from debri so I thought it might be a rust deterrent.

[englands 71]

Undercoat has been used for years and worked very well. However it gets dirty and hard to clean. If you like the look and can handle the dirty look after awhile I say go for it.
If you're stripping it , painting the bottom of the cab and doing a detailed job , why cover it up ? I painted the bottom of my 67 red but my 71 I just sealed and painted it flat black.

[69chevy lwb]

Good undercoating is fine but expect it to become dirt colored. I prefer por15 and I use 3m aerosol undercoating for the places you just can't reach like the insides of cab corners ect. It is possible for the undercoating to cause problems. I have a utility bed that was undercoated and the whole bottom is rusted away. When you peel the undercoating back it's full of dirt and rust. But who knows how it was prepped before hand

[sboris]

Originally Posted by 69chevy lwb

Good undercoating is fine but expect it to become dirt colored. I prefer por15 and I use 3m aerosol undercoating for the places you just can't reach like the insides of cab corners ect. It is possible for the undercoating to cause problems. I have a utility bed that was undercoated and the whole bottom is rusted away. When you peel the undercoating back it's full of dirt and rust. But who knows how it was prepped before hand

Im all in on the por-15. Its non porous. Good stuff. Is what i used, and they make an aerosol now for those hard to reach areas like mentioned above.

[olebluegmc]

Originally Posted by sboris

Im all in on the por-15. Its non porous. Good stuff. Is what i used, and they make an aerosol now for those hard to reach areas like mentioned above.

I'm planning to use raptor liner for the entire bottom of the cab as well as the floor inside, I love the look and I've heard great things. I think I'm gonna go for it and use the 3m aerosol can to coat the inside of my cab supports, cowl ( while I have it open for repair), and any other areas I won't be able to get with the rapor liner

[Jrainman]

I would use short strain fiberglass( cat hair) on your floor patch seams as a base this will fill and seal all your small gaps and pin holes then prime and top coat with your choice

[boynton]

The only parts on my one owner short step that weren't rotted away were under coated. If you get your parts nice and clean hit then with some primer weld everything up and a second coat of primer and then undercoat. Make sure you get every area covered and covered well. Where you see problems is where a section doesn't get covered and then the rot starts. Undercoating is a must if you're going to drive it.

[mike16]

under coating was good...even 43 years ago in AZ. so it cant be bad for the next 43 years either.

inside the cab... On a hot day? things could get kinda messy

[Xeen]

Buy a small can of POR-15 and coat liberally and it likely won't ever rust again.

[dfwright]

I will say that the areas under the bed which had good covering of the original undercoat, the metal under it was very clean. It was a bear getting the stuff off, but it convinced me of the value. Unfortunately, the application of the factory undercoat was "haphazard" at best.