This subject was discussed to some extent in anothe rthread, I thought it deserved a thread of it's own.
So here goes.
Every once in a while, there comes a need to repair a leaking gas tank, or you need to use heat generating tools near a gas tank. Please read this before starting such work.
1) repairing a leaky tank:First rule of thumb: If it's the tank body that's leaking, I wouldn't recommend any repair. Get a new tank, they are relatively cheap and easily available for most aircooled VW models. Reasons for buying a new tank:
- They have no internal rust. What an excellent opportunity to fix THAT problem as an added bonus. - The old, leaking tank may be leaking from one spot right now. You patch it up, and the tank is good for another year or two. Then a new leak will occur somewhere near the first. This keeps on repeating throughout the remaining lifespan of the tank. You hated doing it the first time, and you will be egven more frustrated to do it again. - The new tanks have no external rust, just shiny metal with some crappy "storage" paint on it. The new tank will be dry, and absolutely free of any gasoline vapors as gasoline has never been in it yet. PERFECT opportunity to weld (actually brazing would be better) in a second outlet at the bottom, either a larger outlet for the possible "monster engine" that needs more fuel flow from the tank, OR simply an outlet for a as heater. No need for leak prone tee fittings. If you don't need the outlet right away, you can plug it. But it's good to have it there if your future plans could benefit from it. Now, sand or grind off the "storage paint" with powertools, and don't worry about creating sparks. No gas vapors :) Then paint, rust proof and coat the rust free tank with whatever chemicals you choose. Inside out if you like. The damn lump will last almost forever.
Ok, so you can't afford a new tank. You still have a leak in your old tank that you want to patch up. Forget about chemicals, tank repair putty etc. They cannot offer a loooong term solution. What I see as the only feasible long term solution, is removing old rusty metal and replacing it with new. This means welding. Welding a used gas tank is very interesting. And HIGHLY dangerous too. You can rinse the tank out 10 times with water, but it will STILL have gasoline vapors inside. Just take my word for it. I've been there so many times.
Here's the trick: A FULL tank will not explode in your face. Not even if it was filled with gasoline. Gasoline doesn't explode. It just burns with a flame. gasoline VAPORS however, are highly explosive. That is what an EMPTY tank will have inside. Even after you washed it out with water several times. Unbelievable, but true.
What you should do, is rinse the tank with water several times at first, then plug all holes and fill the tank with water. There will then be no ROOM left for vapors inside. You are planning to weld a patch at the bottom most likely, right? Then keep the bottom side of the tank up, and plug all other holes and fill it up the best way you can. You can use tin snips to start cutting the rusty part out, after the tank is mostly filled with water. (minimizing accident risk, even tin snips might cause a tiny spark). Fill the tank to the brim once you have a hole cut through the leaky spot. Cut out all the rusty metal around the leak spot. Sometimes this means cutting off a chunk of metal several inches of diameter, to patch up a pinhole leak. Then you can weld a piece of sheetmetal over the hole. You can make an overlapped joint in this case. It's more rigid. Water will be sloshing about right underneath the area you are supposed to weld. You need to balance and support the tank so that it doesn't move, because you don't want water to splash out on the welded area. Due to practical reasons, you will always have an air pocket underneath, of some size. Direct water contact to the weld will overcool the weld and it won't hold as well. This air pocket, however small, and regardless of what part of the tank you are patching up.. WILL have gasoline vapors in it. Sometimes there's not enough of it to catch fire or explode, sometime there is. In MOST cases, what you get when you start welding, is a small, cigarette lighter type blue flame that shoots out from a small hole or crack between the new piece of metal and the tank. You might actually want to use a match to "burn off" the vapors beforehand. More vapors will develop while you work, and the tiny blue flame might still erupt from for example a hole you just burned on the thin tank bottom with your welder as you were finishing up the last inch of seam. I've seen it happen *every time* I patched a fuel tank, following this exact procedure. Don't be alarmed, it's normal. If you have minimized the air pocket size inside as I instructed, there will not be enough volume in there for a big explosion. Possibly the flame only lasts for a second or two, and then it dies on it's own. Sometimes it might live longer. Stop working and let it burn on it's own, until it dies off. Most likely you then have burned off all the remaining vapors from the tank. For now.
Finish your job and drain the tank, get all of the water out. Open all passages and remove plugs, allowing air to get inside the tank to dry it out. Two things will happen almost within minutes: The inside of the tank will start rusting, and gasoline vapors will start forming AGAIN, "out of thin air". Yes there is still tiny amounts of gasoline trapped in the seams an even teh pores of the metal, and it will start evaporating after the water has been removed. You can tell, the damn thing SMELLS of gasoline still. And smell, as you know, is caused by molecules of said substance in the air. (if you can smell it, you are breathing gasoline vapors from the tank... however, even if you can't smell it, it doesn't mean that there ISN'T any.. )
Don't belittle the danger. An epmty gas tank is LETHAL. If the vapors inside are ignited, the tank will blow to bits and anyone near it when it happens, is at high risk of getting severely injured or even killed. (Just 2 months ago I heard of someone who had died like this.. and he was welding a DIESEL fuel tank.. gasoline would be even more volatile)
Thank you for your time.
Jan