I wonder if you could get a spray-on liner in there, much like the ones truck beds use. I'm not sure what kind of material they use, but it might withstand gasoline. The tricky part would be having to spray the whole inside through the little gas cap hole. Might be possible and worth looking into!

Clean the old tank using Electrolosys and not acids.
Go here:
Use a real bona fide power supply that you can attenuate and not just a battery charger.

This is the guy who impressed me the most and he provides a hell of a lot of information little of which am I capable of commenting on. He even teaches you how to build your own power supply.
tinyurl.com/bkl4q


This guy says to use 6 volts
tinyurl.com/38oqhq

Here is some one who thought to tell you about polarity
tinyurl.com/38amh3

Here is another with lots of pix
tinyurl.com/2quuy5

I clean my electrolysis work with Denatured Alcohol 'cause it takes all the water off and won't cause new rust.
You must proceed immediately to a dry room.

Then braze in patches where you have to and use lead and flux to fill in the pits and little holes. Properly bonded using a decent flux the lead will make a solid rust proof fuel tight seal. You can buy bars of lead in various hardware stores or auto places or online like this place: tinyurl.com/6ehy7j

The racing industry is all about using burst resistant bladders instead of relying on a metal tank.
A motor cycle racing place may be able to hook you up with a bladder. if you don't want to trust the restored tank.

Even though you only see some pinholes, the rust may have eaten much more metal, and the good steel will be very thin. I havn't had any luck welding thin rusty metal using a MIG welder (even with steel mix gas). You'll need to find the limits of the bad rust and completely cut out and replace the bad sheet metal so you'll have solid steel to weld on. I think it would be a good idea to have the tank mediablasted inside and out so you can see what you're dealing with.

If you try the MIG route, definately get a bottle with steel mix since the gas will keep the metal cooler and help prevent blow-through. MIG welding is pretty easy, but for sheet metal, you'll have to be very slow and methodical. A continous bead (for me) is nearly impossible. I usually give it a quick zap, laying down only about 1/16" of bead. Wait until the weld cools to a dull cherry color, then give it another 1/16" zap. When you start a weld, always clip the blob of of the end of the wire.

Better yet - Pay someone that knows how to TIG weld.

Assuming you can get the tank structurally sound, you can carefully grind the welds and use a skin of filler to get everything smooth. Maybe you could find an old-timer that knows how to work with lead.

Caswell sells a tank sealant and has some tips on prepping the tank. Check it out:

www.caswellplating.com/aids/epoxygas.htm

Regardless of the factory being dead, if we're talking a straight-up cherry restoration here, you'd reverse-engineer the item in hand to come up with the specs. That's presuming having a fully-equipped metal fabrication shop and the skills to match... which you don't have.

On the topic of replacement... sure you don't want to retrofit an old Bennelli or a JAWA or other euro-made tank which might be adaptable? Incidentally, wrecking yard junk is way cheaper than even the cheapest bill of homemade repair materials. After all, in terms of antique collectible value, we're not exactly talking a 1949 knucklehead here where strict adherence to using stock parts only might actually count.

OK, to answer your question about how to execute a practical and inexpensive repair. This will cost less than $90. Welding would be a costly horror movie lesson. Even a seasoned journeyman would hesitate to take that on in your case. If they tell you different, they're funnin with ya. Never mind the years of experience it would take you to come up to speed to even think about doing it yourself.

We get to the option I'd mentioned above, ... resin. Polyester resin with glass cloth = fiberglass. How to do? You'll need to dismount the tank. cut the tank in two along a crayon-scribed centerline which carefully goes around any existing fill or drain apertures, preserving them. Use a rented sawzall or thin metal cutting wheel on a rented handheld grinder. Open up the tank like a clamshell. Now you can access the insides. Remove all tubing & metering devices. Remove rust down to white metal using rust-removal jell or solution and then 1/4" drill -mounted wire brush (cheapest), or rent shop time to sandblast it yourself. (You don't have the budget nor resources to do the electrolytic bath thing.) Remove paint finish with automotive paint remover jell or solution. Once clean and bright, lay the two halves on workbench, open side up. These will serve as a mold for the glass. Patch rot holes with card taped to the outsides. Lay up resin-saturated strips of 3M glass cloth all inside the tank halves built up to a 5mm thickness, while leaving function apertures open and unglassed. Let it cure. (you might want to practice beforehand on an empty coffee can cut in two, in order to get the hang of laying up glass. Library has books on marine application fiberglassing mends). Clean tank edges of repair material so they fit together again. Now we need to create a joining seam surface to get the halves back together. Build up a 1cm lip of fiberglass all along the inside edge of the halves in order to arrive at a mating surface between the two halves, ..sort of a bead running all along both inside edges.. Think model airplane kit, where the two fuselage halves are joined. Let cure. File back the joining bead square and flat until the two halves mate with a clean and tight seam, surface to surface. Then slather mating surfaces generously with JB Weld, or other epoxy adhesive and mate the halves. Hold the 2 parts together until cured by tying up the work with bicycle innertube strips acting as a clamp. Let cure. Now test integrity of the join by plugging working holes and then sloshing around some fuel inside to see if any oozes. Fix that with epoxy. Fill and shape and form the outside rustrot hole areas with additional fiberglass, then file and sand down until outside looks nice. Refinish with spraycan primer and spraycan automotive lacquer. (library books on this). O yeah, before joining halves, get all your mechanical plumbing back in place. Then remount tank unit to motorcycle.

Hope this helps. Any question?

b!

WAIT!!!
Before trying any suggestion, post the question first on www.vjmog.com.
There are specific products for it, one of them is POR-15 www.por15.com/prodinfo.asp
I've read discussions about an even better product of which I don't remember the name.
The guys at VJMOG restore old bikes all the time.
Good luck

UPDATE: Just found the thread www.vjmog.com/ftopict-248.html

Ciaou Giovanni
Even the best goo in the world will not restore structural integrity to a fuel reservoir that is perforated in places, and like paper-thin swiss cheese in other spots. That would be a futile stopgap temporary-at-best bandaid fix just waiting for a chance on a hot day speeding down the 'strada to rupture uncontrolably all down his leg waiting for a spark.

It's not cost effective for him to import an expensive replacement. The market value of the bike would not be noticably improved with the addition of a tank from Italy at a cost of $160USD to $250USD , shipped. The bike total is worth maybe $1000 max, to be generous.... certo piu in Italia dove e genti qui le piace.

No matter the best sloshing around in it of harsh solutions to remove fuel residue varnishes, you'd need to physically open the tank to make visual inspection, and then to accomplish a clean job of physically removing hardened gunk from crevices, and then phyusically remove the varnish remover.

I agree that all the work of fabricating a synthetic fiber-resin tank is probably not worth it.... but it seems to be a labor of love for him. A cleverly chosen tank from another make could be adapted to fit with maybe a re-positioning and re-brazing of attachment brackets to get a good fit. Cheaper and practical and way less work headache.

The only solution which seems to satisfy function/safety/aesthetics/owner's sentiments/etcetera, seems to be the one I proposed, and that he's agreed to above. Which is to build a functional tank inside of the existing rusted-out shell.

Happy trails
b!