Building tanks with WEST SYSTEM® epoxy
Wood/epoxy composite tank guidelines

By Patrick Ropp

Builders have successfully constructed tanks for potable water, sewage, gray water, ballast and diesel fuel tanks and a limited number of gasoline tanks using WEST SYSTEM epoxy since the early 1970's. The regulatory environment has evolved within the last thirty years and has placed safety restrictions on various aspects of tank building, specifically potable water and gasoline.

U. S. Coast Guard (USCG) regulations, Food and Drug Administration (FDA) regulations, American Boat & Yacht Council (ABYC) standards, and insurance restrictions should be carefully considered by anyone, professional and homebuilder alike, planning to build a composite tank. Tank construction material and construction processes have been investigated thoroughly over the years, and the lowest grief option is choosing a certified tank from a manufacturer. Certified fuel tanks have already been tested in accordance with strict fire and flame regulations, and have proven themselves in the field. Certified potable water tanks have been made to minimize or eliminate extractives.

While professional builders are bound by tough USCG regulations and ABYC standards regarding tanks, backyard-boat builders have the option to follow or disregard them. However, if the builder decides to sell the boat, the federal regulations become applicable because he/she has built the boat for the purposes of sale. A good reference for individuals building boats for their own use and not for sale is Safety Standards for Backyard Boat Builders, COMDTPUB P16761 3B, available from. USCG Headquarters or its web site. (See resources at the end of this article.)

Before using Gougeon Brothers' products in tank construction, we advise that customers review the relevant federal, state and local safety regulations, standards, and recommended marine practices, as well as contacting their insurance carriers for restrictions. A wood/epoxy diesel fuel tank may be an uneconomical choice if insurance premiums increase or if state licensing/federal documentation problems are encountered.

Wood/epoxy tank construction
Once the epoxy leaves our shipping dock, Gougeon Brothers cannot control how the epoxy will be used. Some customers will use, and have used, the epoxy to build all types of tanks. For those who decide to proceed with making tanks, here are some guidelines to help make the project as successful as possible. The key to a durable tank is a thick, well-cured epoxy coating. To minimize long-term performance problems, we recommend the following:

1. Coat all interior surfaces with 5 or 6 coats of rolled epoxy
The inside of the tank should be treated similar to the outside of a hull. We recommend a minimum of 20-mil coverage, which is five or six rolled coats of epoxy, for all interior surfaces of tanks. This is similar to the coverage recommended as a "barrier coat" for the exterior of a hull. After it is rolled on, brush the epoxy out to minimize any entrapped air. Install surge baffles to minimize free surface effect, using generous fillets and giving them the same 5-6 coats of epoxy as the rest of the tank interior.

Any tank openings, such as fill, vent, inspection or clean out, should be oversized and also receive 5-6 coats of epoxy. We recommend that fasteners be epoxy bonded in place to secure any hardware to the tank or in the construction of the tank (See details of hardware bonding in the WEST SYSTEM User Manual & Product Guide.)

2. Use a slightly resin-rich/ hardener-lean epoxy mixture
Carefully metered resin/hardener ratio is critical to any epoxy's performance. In all projects, with one exception, we recommend dispensing and mixing epoxy at the target ratio within our acceptable range. Tank building is the exception to the rule. When mixing epoxy for tank coatings, we recommend a resin-rich/ hardener-lean mixture at the outer limits of the acceptable range as shown in Table 1.

This is because excess resin in epoxy is less likely to adversely affect the physical properties of cured epoxy than excess hardener. When excess hardener is in the epoxy matrix, it cannot fully react with the resin and will become suspended in the mixture. Because amines (in the hardeners) are water soluble, they can potentially leach out and cause odd tastes, contaminates in the liquid, and porosity in the epoxy film, among other performance defects. The maximum resin-rich ratios noted in Table 1 are at the end of acceptable ranges for WEST SYSTEM epoxy. These ratios should not be taken any farther from the target. These maximum resin-rich ratios meet specification to obtain a properly cured epoxy film and to minimize extracts leaching out of the epoxy. When using 300, 301 or 303 Mini Pumps, one way to obtain the resin-rich/hardener-lean ratio within the acceptable range is to dispense the following:

3. Mix thoroughly, using a double-pot method
We recommend using a double-pot mixing method when building or coating tanks. Completely thorough mixing of epoxy is difficult to achieve when resin and hardener are pumped and mixed in one container. A film of resin or hardener will cling to the bottom or side of the container leaving areas of unmixed or undermixed components. This can be readily seen when tinted epoxy is mixed together.

To ensure thorough mixing, first dispense and mix resin and hardener in one container, carefully scraping the sides and bottom. Scraping is key because it incorporates the majority of the single components into the mixture. Second, transfer this mixed epoxy into another clean mixing container. Use a new stir stick to mix a second time. This transfer will eliminate any clinging unmixed material and additional mixing will assure thorough crosslinking of both components.

4. Post cure at an elevated temperature
Post cure at a minimum of 120° F for 4 to 8 hours (after the epoxy has cured at room temperature and can no longer be dented with your thumbnail). You can use a temporary oven or radiant heater to apply an elevated temperature post-cure. A high-wattage incandescent or halogen light bulb can raise the temperature in a tank high enough to do the job. This is often the only way to do an integral tank. However, watch the fire hazard as some light bulbs can get hot enough to cause the epoxy to char or possibly ignite.

5. Scrub cured epoxy thoroughly with water
Scrubbing with water and a scouring/abrasive pad, such as 3M's Scotch Brite™ pad, removes any surface contamination, specifically any potential amine blush that may form on the surface. Amine blush is water-soluble and can be removed with tap water. The scrubbing action agitates the surface to help removal. Solvents don't normally remove the amine blush because of its water solubility. So leave the lacquer thinner, acetone, vinegar, alcohol, and other solvents in the storage bin, and use water. After scrubbing, rinse again with water and dry with paper towels.

Considerations for holding tanks and gray water tanks
Sewage holding tanks are Type III marine sanitation devices (MSD) and are regulated under Title 33, Code of Federal Regulations Part 159 (33 CFR 159)-Marine Sanitation Devices. A Type III MSD is considered a certified tank if it (1) is used solely for the storage of sewage and flushwater at ambient air pressure and temperature, and (2) prevents the overboard discharge of treated or untreated sewage or any waste derived from sewage (33 CFR 159.3, 159.12a, and 159.53). Type III MSDs may not be labeled. For Coast Guard Inspected vessels, the Type III MSD must also comply with 33 CFR 159.97. For the specific tank construction, make sure the internal surfaces are very smooth and that all burrs or wood splinters are removed so they don't cause anything to cling to the surfaces. This should help allow the tank to be cleaned efficiently.