I've been given a K 75C fuel tank. It's got a series of holes on the underside, both sides, caused by corrosion and I want to repair it.
I'm considering 2 methods. The first is to use LUMIWELD, which I have never used before.
The second is to use JB WELD, which should be much easier, I think.
Has anybody repaired a holed petrol tank before, if so, how?
Would anybody care to comment on my choices of repair?
I would appreciate any input. Thanks.

I had a hole in the seam of my tank, which looked small pinhole. I took it to an aluminum welder...the hole was indeed 6mm in size. No problems..welded and apart from the welded section no paint damage. Happy with job andI know it is a permnament job. Good luck.

I used JB Weld on different tanks and it never leaked after.
As a preventative maintenance (?) I applied the JB Weld all over along the front part of the seam.
If you have a pin hole, the aluminum is certainly corroded right beside it and you may want to patch a longer strip to avoid surprises in the future.

Common problem with tanks getting older now.
I've had to repair two now using that two part putty that you can buy for repairing radiators, pipes, etc.
Works ok but don't try to use some that's been lying opened for a while. It does seem to go off and doesn't harden properly.
I'm now thinking that I'm going to use some of the epoxy liquid products that the classic bike guys use to line the inside of the tank.
Prevention being better than cure.
Only concern is that I'll need to make sure nothing gets blocked by it. After all, is a bit more complicated in there than yer average BSA tank.
Anyone else ever use the liquid yet?
Maybe an indication that the brick is morphing into classic status now. 

I bought some petrol tank repair putty from supercheap, made sure it was good for aluminium, cleared the area with a wire brush, pressed it in, well overlapped, and no more probs. I thought about coating the inside of the tank with something but the local mechanic didn't think it was a good idea, because as mentioned, the inside of a K tank is pretty complicated.

As above for my fuel tank repair too.

However...first I just plugged the holes and let that set. Next I covered that with another thin layer, overlapping the first by about 3mm. Left it for a few days to fully cure.
Did that another 3 times, each layer overlapping the one before.

It don't leak no more.

Think I'm still going to have a go at the tank lining.
The biggest concern would be if it blocks one of the pipe inlets in the tank but I don't believe that there are any on the 'floor' of the tank. If all the bits are removed first the only other issue would be to make sure that the well where the pump sits doesn't fill up with resin.
I agree that the putty is a handy fix but if you intend keeping it for a while, I would say that it's almost certainly going to be a regular thing. Have this image of the bottom of the tank covered with epoxy putty.
Originally, I believed that it only rusted at the fuel pump where water could gather. But on my last K it was further back so not sure why it rusts in any particular place. Any theories?
Just have this feeling that as the tank gets older, the bottom is getting thinner, generally!
Good second hand tanks seem to be getting rarer now so I feel that it's something that needs to be tackled soon.
I'll report back to the forum when I do it.
Might need a second hand tank myself, afterwards!

This how I repaired my tank
https://www.k100-forum.com/t8796-fuel-tank-cleaning#104593

Unfortunately since that repair another pinhole has appeared so its important to have a really good poke around to find where the corrosion is before you plug with epoxy tank repair

EDIT: one of the neglected parts on these old girls is the fuel cap gasket and seal. They don't get replaced and so there is water constantly geting into the tank.

About fifteen years ago I repaired a steel Guzzi tank with two-part putty and it held good for some ten-plus years, until it rusted through somewhere else. The original hole never leaked. So, six years ago, I repaired a second tank that sprang a leak from being rubbed by a piss-poor sharp weld and AFAIK that one is still intact. It hasn't had fuel in for three years mind, but the inside is still nice and shiny.

Thanks for all the ideas guys. What a great enthusiastic bunch you are.
I'm still trying to make a decision regarding what to use.
The series of holes has been caused by water sitting in the tank for a long period. I once heard of an additive you could put in the petrol to make it absorb water but I don't know what it's called.
I've come across a repair system using POR15. Having used POR15 paints before with great success I'm tempted to give it a go.
I'll let you all know how I get on, but there's plenty more to do yet. I've been trying to get 6 seized exhaust studs out today with no joy. Left them soaking now while I go out and buy a stud extractor tomorrow.

G'day Jimish , I use methylated spirits to absorb water in the tank , I usually add a cup or so when the tank is a little low then fill up , doesn't seem to affect the running much

John R

Jimish wrote:I've been trying to get 6 seized exhaust studs out today with no joy. Left them soaking now while I go out and buy a stud extractor tomorrow.

Why do they need to come out? Even with a stud extractor you stand a very good chance of snapping one or more and then it's an engine-out-and-strip job. If it's just rust on the studs, clean them up with some 800grade wet'n'dry and then run an 8mm die down the threads. Re-assemble using stainless steel nuts and a good dose of copaslip.

+1. Metho is a good method. It will combine with more than its own volume of water and still disolve in petrol.

But if you are going to repair your tank probably best to dump the fuel. Run the fuel right down first, remove the tank and empty the remaining fuel/water/crap. Probably best to remove the fuel cap (its only 4 screws). Tip in a litre of metho, give it a good swirl around, let stand for a few hours, then shake and empty. Epoxy doesn't like metho so make sure the repair area is completely dry.
Replace the fuel filter while you're at it.

I'm with Ainsjac on the tank repair. Had leaks on both sides of my K75 tank at the lowest point when I got the bike, took it to a guy who could aluminium weld it. Job done permanently.
Cheers

I welded the tank on left underside right under the pump. Friend of mine did it with permanently cooling sides of tank with water. so we saved the upper paint.
Now I am suspicious about right underside of tank. I think the only way to save tank permanently is to mount some kind of vent on the lowest section on tank to drain water from time to time? Did anyone try that?

Regards

I should have explained that the engine is on the bench because I'm doing a total restoration on a very sick and neglected K75RT, eventually turning it into a K75C.
The head is coming off for a decoke and to check the valve seats and guides, so the exhaust studs have to come out ( or what's left of them! ) 
A great tip to use Meths to absorb water, I'll do that regularly when the bike is back on the road. I've had to send away for the stud extractor so the studs are still soaking in penetrating oil. I will be using plenty of heat too, but I f they shear off then I will just have to drill them out. I've had lots of practice on this bike!