Gas Tanks:  Repair and Restoration

by Paul Higley

The last gas tank I had that leaked was in my English Ford Cortina GT.  A wonderful car to drive but the tank rusted from the top down due to my habit of putting wet SCUBA diving gear in the trunk.  The trunk floor was the top of the gas tank.  I sold the car rather than repair the tank.  One of my many mistakes.  So other than salty water on gas tank top, or for northern climates where road salt can rust a tank from the outside, why do we have a problem with tanks rusting?  The most common problem is condensation of water vapor collecting in the bottom of the tank.  Warm humid air enters the tank as we drive during the day.  The air drops this moisture at night when it cools and the moisture collects at the bottom of your gas tank.

There are repair kits for restoring even a very rusty gas tank by coating the inside of the gas tank.  Looking into this I found three types of kits to repair the tank.  The least promising is the addition of fiberglass to the outside of the tank.  Should this be your method of choice, this is not the article for you but at least make certain that you use an epoxy resin that is compatible with gasoline and not a polyester resin.  Polyester is the most common resin used with fiberglass and will be quickly degraded by gasoline.

The other systems use either a one part polymer coating or a two part epoxy coating. These are listed as resistant to long term exposure to gasoline, diesel fuel, and additives like methanol and ethanol.  Preparation appears similar for all 6 or 7 brands I investigated including both one part or two part coatings.  The recommended order of events includes using a solution to clean old gas and varnish, a metal preparation treatment and application of the sealant.  In all cases remember that gas tanks with flammable vapors are explosive, so make certain you do not have flames, sparks or heating elements where they can be exposed to flammable vapors.  For any cleaner you buy, it's best to follow the manufacturer's recommended procedure.  Holes bigger than 1/32" or areas so corroded to have lost the mechanical strength needed in a fuel tank should be repaired with new metal prior to coating.

Cleaning:  As with most repairs, it is the preparation that is 90% of the effort.  The goal here is to remove all old varnish deposits and any rust that has formed in the tank and leave the interior ready for the coating.  Gas tanks can have baffles that complicate these efforts.  The first step includes removing the tank from the car, removing the gas gage sending unit and all connections to the tank and draining of old gas.  This is followed by flushing with solvents to remove traces of gas, old gas deposits and varnish. Recommendations included several types of solvents and stripping agents.  The most common was a "Marine Clean" and is included in several of the tank repair kits or is sold by auto supply and boat yards.  Also recommended were paint thinner or dish detergent and water.  Several shops specifically recommended against caustic tank cleaners.  These are difficult to completely neutralize and can reside in the folded and welded edges of our British car tanks.  It may be that the folks at Metal Rehab offer a good cleaning capability for a fuel tank.

Interesting methods to help the cleaner remove thick encrusted deposits include putting hard objects in the tank with the cleaner.  Use these on only water based cleaners as they may cause sparks.  Some recommend a length of chain, or steel shot and one commercial tank repair shop says they put in a hand full of dry wall screws.  I would find the chain the easiest to remove but poor at getting into corners.  A tank with baffles would not be a good one to try a length of chain.  I would think the chances of getting the chain irretrievably stuck in your tank to be unacceptably high.  The steel shot might be difficult to remove and know you have it all out of the tank.  If you have thick soft deposits and the steel shot becomes embedded in the deposits you could end up with a real mess.  The dry wall screws would be good at getting into corners and I suppose you could count them going in and coming out to know you have them all.  Strays may be recoverable with a magnet on a string.

In any case, the cleaner you use must be completely removed once the job is done.  For water soluble cleaners there are two methods recommended.  First flush the cleaner out with lots of fresh water and pour out all the water possible.  The commercial repair shops flush out the water with MEK as it will absorb about 2 or 3 ounces of water per quart. Acetone will remove many times more water for the same amount of solvent and is less expensive.  I have two problems with either one.  The first is the hazard of putting a flammable liquid in the tank and the second is disposal after use.  Should you use MEK or acetone make certain you do not have any loose metal pieces inside that can cause a spark.  If you use the water based cleaners, I think the best procedure is drain as well as possible followed by ventilating the tank with a hot air gun.  The longer the venting the better.  Any residue of either water or solvent will ruin the job.  Take your time here and let it dry well.

Preparation of the metal:  This step is used to remove any remaining surface rust and leave an ideal bonding surface for the tank coating.  Gas tanks are not generally heavy gage metal and sand blasting or even bead blasting may weaken the metal.  Better living through chemistry may be the answer here.  Most kits I found included a "Metal Ready" metal preparation.  There are various brands but they appear to all be a rust stripper that leaves a zinc phosphate coating as a metal preparation for the sealer.  About 12 to 24 ounces of this thin liquid should do a typical gas tank.  Seal up the openings, pour it in, rotate the tank so the solution has coated all surfaces and pour it out.  Shaking may be OK here.  It is best to have a thin coating and all remaining liquid drained.  This needs time and ventilation to make certain the interior is completely dry.  One commercial shop routinely drills a small drain hole that can be plugged by the liner coating.  Your choice, but I don't like adding holes to a tank or drilling into something that may have explosive vapors inside.

Applying the liner coating:  Two coats might be better than one as not only do you get a thicker coating but you are likely to cover areas missed in the first coating.  One quart may be sufficient for most small LBC tanks.  The liner material is thinned with MEK and you should again be careful not to have any source of sparks.  Make certain you have removed anything you don't want plugged or coated with tank sealer.  For a 12 gallon or less tank pour one quart of the sealer and cover the last opening.  Tip the tank around in all orientations and slosh the coating around to completely cover the inside.  It is best to rock it around to coat the surfaces.  Shaking can result in splatters but not complete coatings.

For the one part coatings you should drain all the excess and put it in a covered container.  It can be reused for the second coating.  The two part coatings are not as critical to drain all the excess as they will cure regardless of coating thickness. You should drain but cannot reuse the excess from a two part mix.  Remove all covers and plugs from the tank and allow free air flow through the tank for at least 1 to 2 days.  Full cure on some epoxies can take 7 days.  Warmer temperatures reduces this time.  If the coating is not completely cured before gas is added, the curing process ends.  Do not use open flame or an electric element for drying or an explosion may result.  The single part suppliers recommend not forcing air through for about an hour following application to prevent bubbles in the coating.

The most common problems with tank repair are:

1. Not completely drying the tank of water before coating, none of the liners will stick to wet metal.

2. Not leaving enough time for the sealant to completely cure.  You can't wait too long. The longer the time allowed the better the cure.  Also make sure it is at least 60 degrees F during the cure.  Too low a temperature will prevent a complete cure.

3. Leaving too much material in the tank when using single part liner.  These liner materials don't cure if they puddle inside the tank as they require air to dry.  Thick material skins over and prevents the deeper material from curing.  If it does not completely dry it can get into the gas and form strings or flecks that may get past the filter and plug carburetors or injectors.  Keep rotating around and draining the tank.  Puddles are most common behind baffles or in corners.  This is not a problem in two part coatings as they can cure in thick layers.

4. Not completely mixing a two part coating will cause incomplete cured material and can cause the same problems as above.

Other interesting comments along the way...  Acid metal strippers ARC or "boiling out" solutions are used to remove oxides from gas tanks but they can strip about 1 mil of metal thickness every 15 to 30 minutes of exposure.  Complete flushing is difficult especially for inside seams as in British car gas tanks.  I would steer clear of these.

The single part brands found included:  KBS (claimed as the gold standard), Red-Kote, RENU (Moyer), J.C. Whitney's "Gas Tank Repair Sealer" and the most common POR-15.

The only two part version was PHENOL NOVOLAC Epoxy Gas Tank Sealer from Caswell Auto Products.

On the technical side...  My sister-in-law, a polymer chemist, tells me that although both probably work well, the one part polymers may have surfaces that are less robust to chemical attack than the 2 part versions.  Oxygen in the air slowly depletes the free radicals in the polymer that are necessary to turn the liquid polymer into a hard permanent surface.  The difference may only be a discoloration of the surface of the one part coating when there are particularly nasty additives in the gas.

On the humorous side...  A J.C. Whitney ad for the gas tank sealer says "Not for tanks using alcohol-enriched fuel" and in the same ad "Alcohol-Resistant Gas Tank Sealer, Unaffected by alcohol additives or alcohol-enriched fuel."  Maybe I'll use another brand.

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