Delaminated tank sealant

This topic has 5 replies, 5 voices, and was last updated 4 February 2026 at 11:08 by Dave Halford.

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2 February 2026 at 22:14 #835559
Steve Rowbotham
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@steverowbotham77083
Hi, in a slight departure from Model Eningeering I have bought a part built Steam Traction World 4″ Road Loco – the original build started in 2019 and had been progressed as far as a rolling chassis. I’ve checked the assembled items and remaining kits and as far as I can tell its all there, but have now encountered a problem for which I’m sure there will be some informed views on this forum.

The STW recommendation is to use POR 15 petrol tank sealant to protect and seal the Tender and Belly tanks, which had been followed by the original builder. It is interesting that there is much discussion on this subject on the STW forum, with others suggesting the use of Iso Flex or Cromapol waterproofing products in preference to the petrol tank sealant.

The problem I have is that the sealant on the bottom surface of the Belly tank has lifted – the other surfaces seem to be OK. I have spoken to the sealant manufacturer who has suggested reasons why this may have happened, but stated that I can’t just apply new sealant to the bottom as it will not bond to the old sealant on the sides. They sell a stripper which would remove the old sealant if needed.

The tank has an oval access cover on each side. I have removed one of the 2 ‘bulkheads’ in the tank and now managed to pretty well removed the old sealant from the bottom, so am now happy I could thoroughly re-coat the bottom with ‘something’ via the 2 access covers. The question is what to use that will work with the existing sealant.

I can’t use more of the same Petrol tank sealant as it won’t bond to itself.

Steam Heritage Supplies sell a product called Apexior 3 specifically for such applications, but the data sheets state it will only bond to Steel or itself.

The waterproofing products suggested on the STW forum (Iso Flex liquid rubber and Cromapol fibre re-inforced liquid acrylic resin) are for waterproofing roofing etc, I don’t think intended for long term immersion.

It seems to me the best solution would be strip the old stuff out with the tank stripper and reseal with an actual water tank sealer such as Apexior 3, but not a task I relish.

The next best option seems to be to coat the bottom with Iso Flex or Cromapol and hope for the best.

I would value the wisdom of others on this forum.

Steve
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2 February 2026 at 22:48 #835574
noel shelley
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@noelshelley55608
Does the tank leak ? if not then why line it, just drain after use and store dry ? Unless it is bare metal coating it with anything is not likely to stop rust. Hot dip galvanising would stop the rust of the whole tender and seal it – IF there is not to much distortion. What about making a butyl rubber liner ? Just a thought. Noel.
3 February 2026 at 00:36 #835584
Paul Kemp
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@paulkemp46892
Steve,

The thorny problem! I used isoflex in the back tank of my 4” Ruston Proctor SD. When the 10yr boiler inspection came due I was deep into building the 6” Samson and I didn’t want to spend time on the Ruston so it got laid up, at 10 years the coating was still fine but I have not looked at it since, it’s been dry stored probably 5 years or more now! However I wouldn’t recommend isoflex, it’s horribly messy to apply and a fairly rough finish. Because it’s black it’s hard to see how it is performing and what crap you may have collected in the bottom! I also know people that have had it come off similar to your POR coating.

With the POR15 I know several people who have used it and probably half of those have had the same problem you currently have. I think the secret is getting a good profile on the steel (roughness) for it to bond too and of course making sure it absolutely grease free.

With the Samson firstly due to the cost (it’s quite a big tank) and secondly because I have access to a marine coating company I chose to use a 2 pack solvent free epoxy coating they recommended (and donated). I roughed the surface with very course abrasive and washed it thoroughly with automotive panel wipe. It went on much easier than the isoflex, 2 coats max 24hrs between and it has a glass like smooth finish and it’s light grey so easy to inspect and clean. It’s been on 2 years now and no signs of rust. I was told it is also used by water companies to coat the inside of steel pipes, unfortunately I can’t remember what it was called! However, if you can speak to anyone in the marine coatings business I am sure you will find an equivalent, we used to use baltiflake then made by international but other flavours available on steel hulls which is a glass flake 2 pack, commonly used on icebreaker bows of ships. If I ever do another one I would go this route again.

I have heard reasonable reports on apexior (I think it’s made by Dulux, if they still exist?) and I have heard it has been used on the inside of boilers! Whatever route you choose if using a paint type coating preparation is key, good rough profile and absolutely oil / grease free! Good luck.

Paul.
3 February 2026 at 08:38 #835603
Les Riley
Participant
@lesriley75593
I have a 4″ McLaren. When I built it (2004 -2015), I did the research on sealers and went for POR15 in the tank and tender. I used the supplied cleaner and primer as instructed before using the actual sealer.

My tanks are still as good as new, no rusty water ever comes out of the drains. In fact I still have a dollop of the sealer stuck to my garage floor where I spilt it. It has never worn off!

I intend to use the same stuff on my current engine tanks when completed.

I suspect it is as Paul says, not enough preparation before application.
3 February 2026 at 21:12 #835839
Steve Rowbotham
Participant
@steverowbotham77083
Thanks guys for your helpful responses. The main risks of buying a part built kit are a) completeness and b) standard of previous workmanship; fingers crossed I have all the bits, I have spent many hours checking, and hopefully this is the only workmanship issue I will have to deal with.

If starting from scratch I would use POR15 (after the appropriate prep) as it does seem quite good (though not really designed for water tanks), or a specific water tank sealer such as Apexior 3, but my problem now is the tank is fully built and painted, and to get it apart for cleaning out and re-sealing I would need to grind off 100+ fasteners which are covered in paint at one end and sealant at the other.

At this point I think the most pragmatic approach is to test Iso Flex or Cromapol on the delaminated sealant I pulled out, and if it seems I get a good bond then to seal the base to the sides with that. I will then plug all the holes and water test before completing the painting / lining and final assembly. If this approach doesn’t work I will have to strip it down and start again.

To answer your question Noel I haven’t put any water in yet as Im keeping it dry till I figure out what to do, the manufacturers of the current sealant think moisture is one of the most likely the causes of the delamination, and there are a lot of nooks and crannies in the tank. Also it does need sealing for use even if stored dry, the construction means there can be gaps of upto 1.5mm between the plates, which would otherwise only be sealed by paint.

I will report back when resolved for posterity!
4 February 2026 at 11:08 #835913
Dave Halford
Participant
@davehalford22513
Those ‘rivets’ look like threaded ones with nuts on the inside.
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