HONDA CB750 K2

Mark Hay­cock fin­ishes sort­ing the tank.

Classic Motorcycle Mechanics - - CONTENTS -

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!

A re­cent tank – aw­ful!

Thicker rust was un­der­neath...

Lumps of this came out...

Flow­liner costs plenty...

Time to hit the acid, man...

Paint strip­per sol­vent in use.

Bu­tanone: used to bond plas­tic pipes!

What came out didn’t look good.

Rust turns grey.

Tapox turns red.

Form­ing a home-made spout.

Record­ing the tem­per­a­ture.

...and it needs mix­ing.

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