I have never been a fan of using tank sealers, but sometimes you don't have a choice. Recently, the tank on my 1964 Triumph T100SC started seeping at a spot weld under the right side strut securing the tank kneepad. I didn't want to repaint the tank, so the easiest fix was to epoxy coat the tank and seal the leak from the inside.
Fortunately, this is a project within most people's grasp. It's not a difficult task, but careful preparation is key, especially if you have an expensive paint job to protect. Materials needed include bolts or rubber plugs to seal the fuel tap threads, a rubber bung to seal the filler neck, plastic wrap and newspapers to protect the paint, rubber gloves to protect your hands, a container for mixing the epoxy, a mixing stick and the sealer, of course. Finally, a clean workspace with enough room to move the tank around so the coating is evenly applied is important.
When choosing a sealer, make sure it will stand up to the ethanol in modern gas. A Novolac epoxy sealer such as Caswell sealer will do that. Prepare the tank by removing anything you can unscrew from it, such as badges, filler cap, fuel taps and kneepads. If you use bolts to seal the tap holes, wrap them with Teflon tape. If you use rubber plugs, push them in firmly. You don't want any epoxy leaking out.
Most people seal tanks because of rust issues. Although Caswell says you don't have to remove the rust first, I'd rather start with a clean, rust-free surface if at all possible. I used Evapo-Rust to clean the surfaces inside the tank. Evapo-Rust is non-toxic and won't dissolve your paint, plus it does a bang-up job of removing rust. I just filled the tank and let it sit for 12 hours. I drained the Evapo-Rust back into its jug (you can reuse it until it stops being effective), then blew the tank dry with compressed air.
Once you have the tank cleaned and the fuel taps plugged, it's time to wrap the tank. First, wrap it with plastic wrap. Make several passes, completely covering the tank, including the filler neck. You're building a barrier to keep any spilled epoxy from sticking to your paint job. After the layer of plastic wrap, cover the wrap with newspaper, using masking tape to hold it on the tank. Again, cover everything; you're making an absorbent layer in case any epoxy gets through what comes next. Finally, apply another layer of plastic wrap over the newspaper to hold everything together. With a razor blade or sharp knife, make an X cut to expose the filler neck opening. Carefully pull the layers of covering back to the circumference of the cap opening. With masking tape, seal the edges of the covering to the outside of the filler neck. As a final step, after wrapping the tank I rinsed it with a cup of acetone to remove any moisture left from the Evapo-Rust and water rinse.
Make sure everything you need is within reach, because after the next step, the clock is ticking on applying the coating. You have about 15 minutes to work the epoxy before it sets up, and in that time you have to roll the tank to evenly coat the interior.
Make a filler neck cover out of several layers of plastic wrap, or use a rubber stopper big enough to seal the cap opening. If using wrap, have rubber bands to seal the wrap to the filler neck. Mix the two components of the epoxy. There's enough in the kit for two average-sized motorcycle tanks, so I only used half of each component. With the epoxy thoroughly mixed, pour it into the tank. Quickly seal up the filler neck opening and start rotating the tank. The objective is to get all the interior surfaces covered evenly, but the epoxy is thick, and it gets thicker as it cures. Roll the tank around to spread the epoxy, giving it a minute to settle with each major orientation of the tank (top, bottom, left, right).
After about 15 minutes of this the interior should have a good, even coating. Set the tank down and remove the filler neck plug. Turn the tank over and drain any leftover epoxy into a disposable cup. Once the excess is out, turn the tank right side up and remove the fuel tap bolts. Place the tank so nothing will drain out of those holes; you don't want any epoxy plugging them. Let the tank cure at room temperature for 48 hours, replace the fuel taps and any other hardware — and you should be good to go.
This is where it started. If you look closely, you can see the paint bubbling along the lower edge of the kneepad support. Fuel was leaking because of corrosion at the spot weld.
Before lining the tank we removed all the tank emblems, the fuel taps and the fuel cap. To purge the tank of any rust we filled it with Evapo-Rust, letting it sit for 12 hours followed by a thorough rinse and dry. If you still see some rust, fill it again and let it soak another 12 hours.
Plug the holes for the fuel taps. We prefer using threaded bolts with Teflon tape as the bolts will protrude beyond the threads, but tight-fitting rubber plugs will also work well.
With the fuel tap holes sealed, wrap the gas tank several times with plastic wrap, including the filler neck. Next, wrap the tank with several layers of newspaper, followed by another layer of plastic wrap to hold it all together. Note the large rubber plug for the filler neck.
Cut a hole for the filler neck and seal the plastic/paper wrap to the edge of the filler neck. Next, we rinsed the tank with acetone to remove any moisture. Doing so at this stage ensures you won't damage the paint if you spill acetone.
Next it was time to mix our two-part liner. We used an old glass mixing cup, which we threw away when we were done. Pour the mix into the tank and seal the filler neck with a rubber plug or tape, then start rolling the tank.
Spread the liner mix by rotating the tank for about 15 minutes, letting it sit for a minute or so on each major surface (top, bottom, left and right sides). Once you're comfortable the tank's thoroughly coated, remove the plug from the filler neck and drain excess liner out the neck and into a disposable container.
After draining the excess liner, carefully and thoroughly clean the top of the fuel filler neck of any epoxy that might have settled on it so the gas cap will seat and properly seal.
Position the tank so no liner can drip from the fuel tap holes and remove the wrap, followed by the fuel tap plugs. Leave the tank in this position for 48 hours to let the epoxy liner thoroughly cure. Once cured, reinstall the fuel taps and any tank hardware. You're done!