HONDA CB750 K2
Mark Haycock finishes sorting the tank.
Last time we looked at preparing the tank and cleaning it out but before we move on to applying the coating, sometimes you might find the cleaning process troublesome. Photo 1 shows inside the tank of a bike I bought recently. Apart from rust there was some brown/cream substance that appeared to have been poured in. Scraping at the stuff with a screwdriver showed thick rust underneath (photo 2). It got worse, as there were lumps of the same stuff rattling around inside, which were forced out of the filler hole (photo 3). I think this was a product called Kreem and the application process did not work. It would need to be removed. But how? I did try my normal method, which worked (a bit!). I tried dimethyl chloride, which is the solvent used in paint stripper, on a sample in a jar and that did better, but not completely. Photo 4 shows the result after a week. Kreem is a one-part product and the clue came from the manufacturer’s literature that showed Kreem contained methyl ethyl ketone (now known as butanone), so maybe that could be used to remove it? I did have some, as it is used as a bonding agent for plastic pipes (photo 5) and this worked well in the test jar, eventually breaking it down to a soup. I would need a lot more butanone, and found I could get five litres for £24. As you can see on the tin’s label, butanone is probably best avoided if possible and it was all quite time consuming to dissolve the mess, with the substance emerging like the result of a dodgy curry (photo 6). Not good. I later found that acetone (in a household context usually used as nail varnish remover) can also be used but it evaporates quicker than butanone, so the latter is probably more practicable. I think you can see then that a dodgy lining is much worse than no lining at all from a restorer’s point of view. After the subsequent washing out with hot detergent solution and rinsing, you should find the internal surfaces to be clean, but wet. The natural inclination is to dry the inside but you will probably find that by the time it is dry it will already have started to rust again. So instead, I mixed up a treatment solution from one part strong phosphoric acid (photo 7) to one part water, poured it in, swilled it round a few times over the next hour or so and then poured out as much as I could. Take a look at the warnings on the label and remember that concentrated acid is dangerous, so use gloves and goggles. Also make sure that you slowly add acid to water, not the other way round, and mix all the time while diluting it. Now you will find that the metallic surface and any small areas of rust that have remained have turned an even shade of grey (photo 8). After leaving the tank in a warm place for a week or so you will find that the surface has dried and it will provide a good base for the epoxy coating. Now we come to applying the coating and there is a choice of products on the market. I suggest that whichever you choose, the description specifically
assures you that it is proof against current and future fuels that contain ethanol. This will be a two-part product that requires you to mix the parts together just before the application. In the past I have used a product called Tapox, which sets to a darkish red colour (photo 9). It works okay but has the major disadvantage that it requires a constant current of cold air to ventilate the tank as it is setting, and this is not particularly easy to arrange. An airline would be impractical and I thought a hair dryer might work, except mine only blows hot air, which is no good, so the best I could do was to place a fan next to the open filler hole. I also had an argument with a supplier last time after being sent tins that were only just (by two weeks) in date. One plus point for Tapox is it is relatively inexpensive at around £23. I now use Flowliner (photo 10) supplied by Wyldes in Leeds. The price is currently around £49 delivered, so it is considerably more expensive. The product provides coverage for a tank (I assume of average shape) containing up to 25 litres, and the result sets white, which makes it easy to see inside the completed tank. To use it, you open both containers and pour all of part B, which is a clear liquid, into the part A tin and stir it thoroughly with a flat stick (photo 11). As you can see, there is plenty of room in the big tin to do this. If you squash the tin, as in photo 12, you can form it into a spout so you can pour it straight in without spilling. If you do suffer a spill, clear it off the outside of the tank straight away with the use of a strong solvent. Now you can put on the stopper and move the tank around in such a way that it becomes completely covered inside. A method is suggested in the instructions. Just as Tapox has its disadvantages, Flowliner has a quirk that you need to be aware of. Unlike two-part epoxy adhesives you have probably used before, this product gets hot as it sets. The strange thing is that the heat seems to appear just at the point when it solidifies, about an hour after mixing. The thicker the coating, the hotter it gets, so this is why you need to make sure it is even all round. If you get it wrong, the coating can start cracking as it sets and that is hard to correct. I know this from experience – take a look at the temperature it reached the first time I used it (photo 13). I later realised that I should have used a hose on the outside to remove the excess heat. But if you get it right, you can refit the filler cap and tap, wait for a week and you are back in business with a future-proof tank. Hurrah!