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Triumph TR3 - Patching Underbody Rust

Shipwright's Disease Chapter 5

Having now logged several hours under the cleaned-out rear end of my TR3A, I've been able to study and photograph--in agonizing detail--the condition of the underbody and frame. Yes, there are spots of rust through along the bottom edges of the box at the lower rear of the drive shaft tunnel. Since I don't plan to pop off the body any time soon, I'm wondering what my options are to patch up this area to minimize further damage in the short run. Worth the effort?

My plan is to finish scraping and degreasing the underbody from the rear bumper to the rear of the drive shaft tunnel, then paint it using POR-15 and their Metal Prep. I'll do the same to the portions of the frame and shock brackets that are accessible. The same treatment is in store for the springs, axle/diff, brakes, exhaust and shocks. That way, I'll feel really good whenever I look under the car back there.

So...what are my options for treating small patches of rust-through? I've dabbled in light bondo work before, but I thought I would consult the experts before initiating the cover-up.


Bill Stagg
1960 TR3A
Bill Stagg

Bill: In regards to your previous thread, there is a pretroleum product called Kroil (spelling?) They make a pretty serious penetrating oil which I have used before, albeit yrs. ago...
If I can find any resources, I'll let you know.

Perhaps researching some of these "rust stopping" products made for metal might be worth it prior to filling and finishimg? I know some folks have filled in w/ lead or lead-based auto body fillers, some "patch over" w/ aluminum or sheet metal. But this is a splash prone area, so not a bad idea to seal it up fairly well for now, pending how long "now" is going to be. (the omnipresent question)

I've heard POR products referenced before - what type of paints are these?

If you're eventually doing a frame off, Rustoleum rust preventitve paints do work fairly well for the short term. Somewhat less toxic and easier to use while lying in contorted positions...

M.G. '56 TR3
Mike G.

POR stands for "Paint Over Rust". I just got back about 100 TR3A bits and pieces (5 cardboard boxes) from the sandblast shop (7 hrs = $500.00 - a bit steep) and next week, they will be degreased, treated with phosphate (all from POR), allowed to dry, washed down with water, allowed to dry and then spray painted with POR-15. Then it's 4 days to dry. If you get it into the threads, good luck trying to tap the threads later. If you get it on your hands, it'll take a month to get off. If you paint your underbody (no the TR) you may never get it off.

I have made a video of much of the work I've done on TS 81551 L over the last 4 years. All the pieces I needed I had made by a local one-man sheetmetal shop and MIG welded it in myself.

I also have 6 photo albums (before digital cameras) of my TR3A work.

See what Clive is having done in England on his early TR2.;act=ST;f=4;t=1661;st=40

Don Elliott
Don Elliott

So Don, what techniques do you recommend for patching up the little bit of "lace work" I have on the underbody, at least for the short term (read: several years)? I hear your caution on POR for the underbody.

Mike, you may have a good suggestion on the Rustoleum for underbody work. Makes sense.

I'll e-mail you both some pix showing the rust-through about which I'm talking.


Bill Stagg

If you find the tar won't stick to cover the holes (to me they look quite samll), you can cut some patches out of sheetmetal or a tin can and use a pop-rivet or two to holt it in place then tar over the edges.

I think you can buy aerosol spray undercoat tar and spreay it all over this area as an undercoat to seal the joints and prevent water from getting into the overlap joints between the old and new metal - and also between the old sheetmetal patches and the original surfaces.

This should do you for 2 to 5 years. if you check once a winter, you could add a bit more where you see more rust breaking through - when it's all dry.

When I bought "TRusty" new, I had it undercoated as an optional extra at the dealer for $35.00. With time, heat of summer and whatever, the tar split or cracked and these opened up. Then the rain and splashing got into the cracks in this tar-like undercoating and collected down in the "pocket" formed between the sheetmetal and the inside of the tar. It sat there and sat there with no evaporation and eventually I had holes. So check it to re-seal any new cracks or gaps in the tarred areas.

Don Elliott

You could try roofing cement, made to seal leaks in your shingles or metal roof. This stuff is pretty much like undercoating as far as I can tell. You could even use a layer of tar, a piece of screen, and more tar over that. Should last 5 years, and it's cheap, like $3. It'll smell like your spring bushing when you heat it later, though.


Thanks for the tip. I was just wondering about roofing cement this weekend as I prepared to finally tackle the patching job. I may combine it with a pop rivet metal patch as Don described.


Bill Stagg

This thread was discussed between 18/02/2004 and 23/03/2004

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