Classic British Bike Magazine

Caswell tank sealing kit

Two-part epoxy petrol tank salvage system


Motorcycle fuel tank | Ethanol | Repair kit | Fibre glass tanks | Help & Advice


Caswell makes bold claims regarding the effectiveness of its chemical engineering resins. We've got a kit to study for as long as it takes, and we're keen to see if it works or fails. Watch this space.

The problem


We've got a few bikes around here with petrol tanks that have seen better days. Our 1945 BSA WM20, for instance, was restored some years ago by a well-established classic bike mechanic (who, for various reasons, we'll refer to simply as Mr X). A few years back, the internal tank liner on this hardworking sidevalve started to come away from the steel. The first symptoms were a few gritty blockages in the fuel system—notably around the filter gauze attached to the petcock.


It soon became apparent that what was first thought of as general accumulated muck on the move, was in fact the entire tank liner going south. All too quickly, that liner began detaching in large chunks. If you can imagine an large brown ostrich shell suddenly cracking all over, that was pretty much how it looked. So we removed the fuel tank for a closer inspection.


There were no leaks. No pinholes (nothing obvious anyway). No sign of worn, damaged or corroded seams. And no obvious thinning of the metal anywhere (i.e. areas where finger pressure could deform the tank). That, naturally, made us wonder why the liner had been fitted in the first place. Finally we loosely concluded that it had been done as a matter of routine. You restore an old heap. You line the tank. Simple. But not necessarily smart.


Anyway, an hour or so of fiddling around soon had all the pieces of the ostrich egg in a heap on the floor. We should mention that we did photograph this stuff. But unfortunately, we've misplaced the images (they might turn up some day, so come back and check if you're that curious).


We considered using some other fuel tank sealer. But there really didn't seem much point. The tank, as we said, wasn't leaking. The paint was still in reasonable condition (albeit slapped on extra thick by the aforementioned Mr X in what is evidently a very brittle coating of cellulose or enamel—we'll have it repainted some day). And we were curious to see how well the tank would fare after a few weeks or months on the road.


Two or three years on, the tank is still functioning properly. But it's a watching brief because time and corrosion is an implacable enemy. Meanwhile, we're hearing first and second-hand accounts of other riders with fuel tanks experiencing more severe problems. Some have never had tank liners fitted. Some have. And naturally, there's little or patchy consensus on which liner system is best—or, indeed, which system works at all.



Caswell will sell you a tank sealing kit for just under £60. You can opt for a clear, red, blue or black liner if that floats your boat. The firm also market a range of plating kits, paint systems, polishing kits, anodizing kits, degreasing products and more.



Meanwhile, we've got an old 250cc BSA tank and a well worn 750cc Triumph Bonneville tank that we're looking to fit with a tank sealer, purely for experimental purposes. Both tanks have some corrosion issues—and fairly substantial in the case of the Bonnie. We've also got a fibre glass tank that we're thinking of repairing and lining.


Here are the chief tank sealing products on the market—and they're not arranged in any particular order, except to say that the Caswell system that we've recently been sent is the one we'll be fooling around with.












The only tank sealing system we've ever used is Slosh (last on the list), and for the time we owned the bike, the tank sealant held up well. However, that was back in 2011, and we sold the bike around six months later. Despite having asked the owner to keep in contact and let us know if problems have developed, we've since lost touch with the machine.


Ethanol, MTBE, and tetraethyl lead


The biggest problem with modern petrol appears to be ethanol. We say "appears" because we're not chemists or petroleum experts, and there's no doubt plenty of misinformation on the loose to which we don't want to add. So treat everything that follows as (fairly) reasoned opinion rather than hard, indisputable fact.


The ethanol content in UK petrol has increased significantly over the past decade. It's the successor to methyl tertiary-butyl ether (MTBE) which is an anti-knock additive used to oxygenate the fuel and raise the octane number. MTBE replaced tetraethyl lead and has been in the pumps for around forty years.




Tetraethyl lead




Tetraethyl lead was designed to reduce "knocking" in engines. Knocking happens when the detonation of the air/fuel mixture happens inappropriately (i.e. prematurely). Tetraethyl lead also helps cushion valves seats (notably the hotter exhausts seats) and reduces wear and tear. Tetraethyl lead, however, has been fairly conclusively shown to damage human brains, notably in the young. So pretty much everywhere in the world lead has been removed from fuel and replaced by MTBE.


MTBE is made from natural gas and oil derivates. It's an oxygenate which means it adds oxygen to the fuel/air mix and helps promote/encourage a leaner burn. But it's pretty dangerous stuff, environmentally speaking. It gets into the groundwater and kills whatever it can if the concentrate is high enough. However, it helps clean up exhaust emissions, so on paper it can look and sound like a major social benefit—and it also has some medical uses when handled right. Either way, for a long time it was added to fuel, and one way or another you're likely to have come across it.


Ethanol is a safer alternative to MTBE. Basically, ethanol is alcohol. You can drink the stuff neat if you're brave/stupid enough, but you'll probably get burns in your mouth (not to mention other health problems). Ethanol is also an oxygenate, and modern engines can happily run with 5% - 15% of ethanol in the petrol. However, older engines are less happy with it because it attacks rubber components such as fuel lines and seals, and it has an appetite for older fuel tank liners.



How to use Caswell tank liner


Firstly, the Caswell kit will tell you all that you need to know, but we were disappointed by the cheap, two-page photocopied instructions that came with the package. It might all be very informative, but it doesn't do much to inspire confidence in the product—and it looks very amateur.


Consequently, Caswell would do well to reconsider this area and provide a small illustrated booklet with FAQ and suchlike. Confidence, after all, is everything. Most of us can be pretty precious about our motorcycles, and we generally don't like introducing products that look like they've been thrown together.


Beyond that, there's kit looks comprehensive, and well packaged. There's no great mystery with it. The principles are simple. If you've got a failing fuel tank and can't afford a new one or a "proper" repair, you can use a tank liner which is usually resin based. But ultimately, it's like putting a proprietary quick-fix product in your car radiator or motorcycle tyres. Such products/sealants can work. But the only proper fix is a proper fix. Consequently, if your fuel tank has holes or damage, have it repaired by a tin basher, or buy a new tank. Ultimately, the "truth" will out.


Keep that in mind.


To re-line your fuel tank, you first have to clean out any existing liners (or what's left of them). In our case, we had the aforementioned ostrich egg material. Hard. Brittle. Stubborn. Other riders have similar muck in their tanks. Caswell provides a tank seal removal solution that you slosh around inside the tank—having first done whatever you can to protect the paintwork. But if you do suspect you tank has perforation problems or weak seams, you can't really check it properly until you remove the paint.


If you don't have an existing liner, you can skip this stage.


1. Remove the tank and empty the fuel. Keep your cigarettes/blow lamps/fireworks well out of the way. Ventilated areas only please. The yard is fine. Fill the tank with soapy water. Don't get crazy. A mild solution will do. You're not looking for a bubble bath. Seal the fuel cap. Close the petcocks. Agitate the soapy mixture. Leave it for an hour. Agitate it again. Rinse it. Rinse it again. Let it dry.


2. Remove the petrol tap/s and seal the hole/s in whatever way you reasonably can. Blu-Tac or similar is fine. You might want to stick some duct tape over the Blue-Tac to keep it in position. The idea is to prevent the threads for getting epoxied at the next stage. So remember where you're headed here.


3. Protect your paintwork with some kind of plastic film, or a plastic bin liner, or whatever. Tape it carefully. Double protect it if you can. Why take chances?


4. Wear rubber or latex gloves. You can develop a long term or permanent sensitivity to cleaning chemicals and epoxies that will leave you scratching forever and/or will give you more significant skin problems. However, use this stuff thoughtfully and intelligently (and protectively) and there's every probability that you'll do okay. Wear eye protection too. And once again, use this stuff in a well-ventilated area. You don't want to breathe the fumes, but you don't want to get paranoid about it either. Just strap on a half decent breathing mask (forget the really cheap paper thingies), and get some air flowing past you. An electric fan will be fine.


5. Pour in the fuel tank sealer remover and agitate it thoroughly. You don't need to dilute this stuff. Use the whole bottle. Added plasterboard screws will help attack the old liner, but it's the Caswell solution that's doing the clever, techy stuff in dissolving the ostrich shell. You might need to do this on and off for a few hours. Be patient. It will probably work (unless you've got something very unusual coating the inside of your tank, and we can't think what that might be). Most of all, be thorough. Why? Because tank-sealant failures generally happen as a result of poor preparation—and note that none of the products on the market come with a guarantee.




Don't use the fuel tank remover solution on fibreglass tanks. Instead, you'll have to mechanically remove the sealer with elbow grease and scrapers, etc. Why can't you use the solution? Because it will attack the resins in the fibreglass and will dissolve it all into a nasty mess. But why seal a GRP tank, anyway? Because modern ethanol-based fuels attack fibreglass. Simple.


6. If the (steel) tank is rusty, do what you can to remove serious rust. Actually, do what you can to remove it all. Some folk feel that a slightly rusty surface is actually very good inasmuch as it provides a mechanical key for the forthcoming epoxy. There's some truth there. Just do what you can. Then mix up the de-rusting solution if you've opted for that. It's actually citric acid; same stuff you get in lemons and oranges, albeit a little more concentrated. Follow the latest instructions from Caswell (i.e. the instruction in your kit). Usually one litre of solution to seven litres of warm water in correct. Don't use cold water, and don't risk scalding yourself with hot. Just use warm, and as warm as you can stand. The heat helps the chemical reaction, of course. Afterward, empty the solution, and you can filter it for re-use if you want. Tip: don't drink it.


7. You want to store your new epoxy sealer at room temperature for a day or so before use. What is room temp? Well, somewhere between 21-degrees and 30-degrees. But aim for a spot in the middle. The new sealer won't flow around the tank if it's too thick (from being cold), and it won't stick to the sides of the tank if it's too thin (from being too hot).


8. Open the tins of epoxy sealer. It's a two part product, so you'll want to mix those parts together. Some folk try to scrimp and use half of each tin and save the rest. We wouldn't bother with that. The more epoxy you have inside the tank searching for bare steel to coat, the better (up to a point). Mix the contents of the tins in a suitable/disposable container. Two minutes of thorough mixing will ensure that all the epoxy atoms are suitably gratified with corresponding hardening/setting atoms. When you're sure the mix is ready, and when you're gloved, goggled and aerated, pour the mix through the fuel tank filler. Remember that the other holes will still need to be blocked, notably the thread for the petcocks. Remember too that the outside of the tank (i.e. the paintwork) wants protecting. Finally, use cling film or a balloon (or both) to seal up the fuel filler as soon as you introduce the mix.


9. Shake the mixture around. And invert the tank, and keep shifting it through every angle in the universe. Give the mixture a moment or two in each dimension. It might be a little thick, and it will need time to adjust and find all the crevices. Note that if it's too thick to flow, you can add some lacquer thinner to the mix (no more than 5%). But we'd prefer not to adulterate the solution. Instead, we'd make sure that the room temperature is correct by heating the garage (and you can do this stage in a garage; just wear the right breathing mask). We're not talking about mustard gas here. Just a few epoxy fumes that are better out than in.


10. You'll want to spin the tank for a few minutes at least. We've already said that, but it bears repeating. It's no good protecting only the bottom of the tank even if that's only where the rust/leaks are located. The idea is to coat all surfaces that the petrol will come in contact with. You'll have to use your sense of touch to decide if the mix is "doing the rounds", which it probably is. Finally, you need to set the tank down, carefully remove the cling film/balloon from the fuel filler, and invert the tank—taking care to empty the residual epoxy mixture into a disposable container. Let it drain for five minutes or so. Congratulate yourself.


11. Caswell recommend blowing through any ports leading to an internal fuel filter. The firm reckons that 10 minutes of blowing will do. You'll have to make your own arrangements in this regard. It's out of our experience.


12. The epoxy liner will need a day or two to "go off". Then you can refit the tank, replace the petcocks, and add fuel. But if you're in a rush. You can point a hot air heater or something at the tank after you've drained it. But don't use anything with a naked flame. A decent gust of heat will halve the curing time. Generally, our advice is to let it cure for as long as possible.


And that's it. After it's all done, you'll want to save the planet by clearing up sensibly, and you'll want to monitor the tank for problems. Chances are you won't find any, not if you've followed the instructions. But if you do, Caswell would like to hear about it.


Meanwhile, if you've any experience with this kit, be it good, bad or indifferent, please fire off an email with details—and pictures if you've got any.



A few final words on epoxies and procedure


Pretty much all of the fuel tank sealing systems on the market are epoxy based. But not all epoxies (or polyepoxides) are the same. Once again, we don't know what the chemical composition is, and even if we did it probably wouldn't make much sense. But Caswell is claiming that their molecules (or whatever) are better than anyone else's—and so far we've got no reason to challenge that. People do speak well of this system, and it's hard to find reliable evidence to the contrary—although we have found one or two examples of poor practice and procedure.


Epoxies, unlike many other similar materials, provide three-dimensional cross-linking. Think of a piece of lattice work or a decent trellis frame on a motorcycle and you'll get the idea. Generally, epoxy resins are feeble until catalysed. Then the chemical magic happens, and it is a kind of magic. The two components are mixed thoroughly, and then the die is cast. Ten minutes later, or half an hour later, the reaction is well under way and the resin begins "going off". Or setting. Or hardening.


Epoxies that cure at room temperature are less strong than those that cure under heat and pressure. So when you read about aircraft wings being epoxied, it's not the same as a wood an epoxy canoe being built in a shed. However, even the most basic epoxies are pretty impressive and generally offer strong and durable mechanical bonds that are broadly-speaking resistant to many everyday chemicals.


But a word of caution: it's perfectly possible to manufacturer a cheap epoxy that barely passes muster. So just because the word EPOXY is on the tin, it ain't necessary worth the asking price. But cheap, get cheap. That's generally how it works. To quote an old advert from the 1970s or 1980s, some things are reassuringly expensive.


We'll report on our further experiences as and when. Good enough?




Think you know better? Good, drop us a line at Sump and we'll include your thoughts/experience.



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