Repairing plastic fuel tank

I have a small outboard motor with a split gas tank. The tank holds less than a quart.
I've repaired this by welding a thick plastic sheet (the sort used in
plastic fuel cans) over the base of the tank so that it seals all the way round, making the tank gas tight again. Its an ugly repair, but I filled the tank with air to 30PSI and submerged it in water - no air leaks. I've given each side of the tank a good crack on the bench, re-filled it with air and still no leaks. Can I now consider this a safe repair?
TIA
Steve
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Steve wrote:

I'd finish off by wrapping the outside of the tank except the filler and outlet and maybe the mounting points with a layer of glassfibre tape and epoxy resin. That will put the strength back if any part of the weld is dodgy. You will need to sandpaper and solvent wash the outside of the tank as the plastics used for fuel tanks typically dont bond well.
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There would be a problem there. The tank is probably polyethylene and epoxy does not stick to poly well at all. It sticks so poorly that I use it for a release in form work when laying up flat fiberglass panels.
OTOH, the tank probably cracked because of poor support and vibration. IT might be helpful to mold a tray in FRP to support it better.

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Glenn Ashmore wrote:

I seem to remember a procedure to get epoxy to bond to difficult plastics (including IIRC polyethylene) which basically was degrease, sand well then extremely quickly run a blowtorch over the surface. I remember it was important to move the flame quick enough for the surface not to actually melt. This was supposed to leave a chemically active surface that epoxy would bond to if it was coated promptly. I hesitate to reccomend this for a fuel tank :-)
I also suspect that the bond would be poor at best, but if there is a epoxy GRP shell round it and its well sanded to mechanically key the surface it should prevent or at least contain any catastrophic failure. Your point about making a supporting tray is very good as well although I would personally incorparate it as extra reinfocement underneath in the epoxy grp wrapping.
I think that the OP wanted to hear something along the lines of 'it will be fine, go ahead and use it' but even if the OP had made a career of welding plastic the best advice would be bin the old tank and replace it.
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Glenn Ashmore wrote:

Yes, I would NEVER reccomend trying to patch a hole in a plastic fuel tank with epoxy for exactrly that reason and gas and two stroke oil would make that adhesion even more uncertain, however the OP has got it gas tight and the existing tank should function OK as a gas tight liner in a GRP jacket. I forgot to mention USE COARSE SANDPAPER. I think the bond will be poor, but with it right round the whole outside of the existing tank except the neck and outlet, there is a very large bond area and very low stress on it.

You are probably right about the poor support and using more than one layer of tape over the whole tank and several layers of glasscloth underneath would be a GOOD thing.

I seem to remember a procedure to get epoxy to bond to difficult plastics which basically was degrease, sand well then extremely quickly run a blowtorch over the surface. I think I remember it was important to move the flame quick enough for the surface not to actually melt. This was supposed to leave a chemically active surface that epoxy would bond to if it was coated promptly. I hesitate to reccomend this for a fuel tank :-)
I think that the OP wanted to hear something along the lines of 'it will be fine, go ahead and use it' but unless the OP has made a career of welding plastic the best advice would probably be bin the old tank and replace it if possible.
--
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On Sun, 12 Sep 2004 20:01:51 +0100, "Steve"

I would have wrapped duct tape around it and considered it safe. If you mean is it possible it may start to leak again, probably, when is anyones guess. -- Visit my website:
http://www.frugalmachinist.com Regards Roy aka chipmaker
Contents have been checked for spelling. I just decided not to make any needed corrections!
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Hey Roy, Speaking of tanks and such; here's hoping you and the better 1/2 are getting on nicely after the unfortunate "tank" accident you had a while back.
Best regards,
Bob Swinney
wrote:

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On Sun, 12 Sep 2004 20:01:51 +0100, "Steve"
No. You've tested that it's doesn't leak at present (and quite competently by the sound of it) but this demonstrates nothing about the crack's re-emergence if it decides to propagate further. The trouble with welded patches is that they don't do much to help with this.
Secondly, this is illegal to fill by UK rules (actually it might not be, you might be under the volume limit)
Thirdly, petrol tanks shouldn't split. The techniques allowed for making them don't involve seams that would tend to do this. If yours did split, I'd be thinking it was a PoS that I'd be well rid of anyway.
Overall, it's your call. You're probably safe, and you may even be reliably safe. But if it was me, I'd put my efforts into fitting a replacement metal tank, not trying to repair plastic. Plastic just isn't right for this job, and age and UV will only make it worse.
--
Smert' spamionam

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