I initially wrote this thread on another forum but am including it here in case you find it useful. There are certainly other methods that can be used but the ones I've discussed here have all been proven to work long term.
Methods of waterproofing a plywood tank.
1. Two part epoxy paint
The most commonly used and reliable epoxy paint for plywood aquarium builds is Sweetwater epoxy available from Aquatic Ecosystems : http://www.aquaticeco.com/subcategor...aints-1-Gallon
It is completely non toxic when cured and has good adhesion to plywood. This epoxy paint has solvents mixed in and is only 65-72% solids. The solvents mean that it ships as a hazmat product and will let out strong, toxic fumes while curing so a well-ventilated workspace is essential when using this product. It also means that it has a nice consistency and is very easy to work with and apply. Sweetwater can be used as a stand-alone product for waterproofing a plywood aquarium. While several members have done this successfully, others have reported that leaks have developed after long periods of use due to stress fractures forming at seams or pinholes due to falling decor. It is therefore important to have a well built structure when using this product as excessive flex could potentially contribute to leaks. Silicone will adhere well to Sweetwater so when sealing a tank with this method you should first apply the Sweetwater and then silicone in your viewing panels.
Epoxy paint summary
Pros: Easy to apply, available in a range of colors, silicone will stick to it making window installation easy.
Cons: Toxic solvent requires hazmat shipping and well ventilated workspace, does not add structural strength to a build, potential risk of failure in an inadequately supported tank with excessive flex.
2. Fiberglass resin with epoxy paint top coat
Reinforcing a plywood coating with fiberglass is an excellent way to add structural strength to the build and impact resistance to your coating. The cheapest way to apply fiberglass to a build is to use fiberglass resin to wet out the cloth. Lightweight fiberglass cloth and fiberglass resin are available from online vendors and are also generally easily found in “big box” stores like Home Depot and Lowes in the US eg. http://www.lowes.com/pd_73902-63-E77...tId=3143485&Ns
Fiberglass resin is generally polyester resin and requires a small amount of hardener to be added as a catalyst. Effective application of fiberglass takes a little skill but is fairly easy to learn with a little practice. It is important to avoid bubbles in the fiberglass layer which may ultimately pop under water pressures and compromise your seal. Polyester fiberglass resin is fairly cheap but has a very strong smell and requires a well ventilated workspace. In addition, it is not sufficient as a standalone waterproof barrier coating and will leach out chemicals into your aquarium water. Therefore, you must finish a fiberglass resin coated aquarium with a non toxic, waterproof topcoat. Sweetwater (see previous section) is an excellent product for this purpose. Using Sweetwater over a layer of fiberglass overcomes most of the potential cons of Sweewater alone.
Fiberglass resin + epoxy paint summary
Pros: Relatively cheap way to add structural strength and impact resistance to your build, epoxy topcoat is available in a range of colors, silicone will stick to it making window installation easy.
Cons: Fiberglass application requires some practice and skill, toxic fumes in the resin and paint require well ventilated workspace, fiberglass resin is not sufficient as a stand-alone barrier coating.
3. Two-part marine epoxy resin
There are many different brands of epoxy resin available but only true two-part marine epoxies should be used for plywood aquarium builds. These epoxies have a long established and successful history in waterproofing wooden boats. They differ from epoxy paints in that they are 100% solids. Several brands are available but some tried and true options include:
West Systems 105 (one of the more expensive): http://www.westsystem.com/ss/epoxy-resins-and-hardeners
US Composites (cheaper option): http://www.shopmaninc.com/epoxy.html
Max ACR (newer, relatively affordable epoxy being marketed specifically for aquariums): [Ebay Link Removed] (search for the seller “polymerproducts” on ebay)
There are many other marine epoxy brands out there that you could use but these three cover the spectrum of price and have all been successfully used in waterproofing plywood aquariums. Marine epoxies come as a resin and a separate hardener that have to be mixed in a precise ratio. It is best to use slow hardeners when sealing a tank with these products to give you a longer working time and better penetration into the wood. The West Systems use guide is an excellent resource to learn how to properly use marine epoxy: http://westsystem.com/ss/use-guides/
I strongly recommend reading every section of the guide before beginning to work with these products.
Marine epoxies can be used as a standalone product to provide a completely waterproof and non-toxic coating for a plywood tank. However, they are fairly brittle when cured and can be susceptible to stress fractures at seams and damage from impact which will compromise the barrier coating. Please see http://www.jonolavsakvarium.com/eng_...t/article.html
The best way to avoid these issues is to incorporate a layer of fiberglass cloth into the epoxy resin. Epoxy resins can be used to wet out fiberglass cloth in much the same way as polyester fiberglass resin but offer several advantages. There is not strong smell, the cured resin layer is completely waterproof and non toxic and is slightly less brittle than polyester resin. With the exception of cost, epoxy resin is an all around better option than polyester fiberglass resin. As discussed above, wetting out fiberglass cloth does take a little practice but with a little skill is an excellent way to add significant structural strength to your aquarium.
When used with thickening agents such as colloidal silica, marine epoxy can also function as an excellent adhesive, particularly for slightly loose joints or joints where you cannot deliver high clamping pressure. It can therefore also be useful as a waterproof adhesive during the construction and assembly of a plywood aquarium.
Silicone will adhere well to epoxy resin so waterproof your tank first and then silicone in your glass viewing windows. Epoxy resins can be tinted if you want a colored coating but will usually still show some wood grain. Sweetwater epoxy can be used as a topcoat if solid color is desired.
Two part marine epoxy summary
Pros: Can add significant structural strength to a build, particularly when used with fiberglass. Minimal smell. Effective standalone waterproof barrier layer. Silicone will stick to it making window installation easy.
Cons: Expensive, can suffer from stress fractures if used without fiberglass on an inadequately supported structure.
4. Pond Shield
Pond Shield is a 100% solids two part epoxy resin available from Pond Armor http://www.pondarmor.com/
It is different enough from the marine epoxies described above that I thought I should give it its own section. Pond shield is non toxic (so no Hazmat shipping is required). It has practically no odor and is safe to apply indoors. Pond shield is an extremely thick epoxy which can make it somewhat challenging to work with. The black pond shield is the thickest and has a consistency slightly thinner than honey. It can be thinned slightly with denatured alcohol to make it more workable, but thinning also increases the risk of not getting the required thickness in a single coat. One 1.5qt kit of Pond Shield claims to cover 60sq ft at 10mil thickness. For many applications a single coat is sufficient to get a 10mil coat that is completely waterproof.
Pond Shield is best suited to waterproofing well-supported structures with no flex. If you have any concerns about the integrity of your seams it is best to fiberglass them to prevent the formation of potential stress fractures. Pond Shield is ideal for sealing concrete tanks. While it adheres to wood it adheres even better to concrete so one option when using it on a plywood tank is to first line the inside of the tank with Hardiboard. If you instead choose to apply it directly to a wood aquarium it is best to first do a light wash with 30-40% alcohol-thinned Pond Shield to get better penetration into the wood and follow this with a single coat of unthinned, or very slightly thinned Pond Shield.
In my personal experience I have found that the thick consistency of Pond Shield can make it challenging to work with and results in several areas that require touching up after the initial coat. Careful inspection and touch up is critical to the success of using this product.
Pond Shield summary
Pros: Completely non-toxic and odor free. Available in a range of colors. Requires only a single coat and touch up. Silicone will stick to it making window installation easy.
Cons: Thick consistency can make it difficult to work with. While it remains somewhat flexible after curing it is susceptible to stress fractures in poorly supported structures.
5. Liquid rubber
These waterproofing products are elastomeric emulsions that remain highly flexible after curing. While there are a number of these products available, some examples of those that have been successfully used with plywood aquariums are
Zavlar / Permadri Pond Coat (in the US) http://www.permadri.com/pond-coat.html
and Ames Blue Max http://www.amesresearch.com/bluemax.htm
These products have minimal odor and are easy to apply. Zavlar/Pond Coat requires several coat to achieve a 40mil thickness that provides a waterproof barrier. One gallon will cover 30sq ft at 40mil thickness. The primary advantage of these products is that their incredible flexibility, which allows them stretch and resist fractures in inadequately supported structures. However, it is still a good idea to use drywall tape on seams while applying these products in order to to reduce potential stress on the coating at these spots.
Zavlar/Pond Coat will initially appear brown and will dry to a black coating. However this black coating will gradually turn back to brown after being submerged for a period of time. Ames Blue Max dries to a translucent blue color.
Zavlar/Pond Coat will not cure when applied over silicone. Similarly, silicone will not adhere to cured Pond Coat. This incompatibility with silicone is a disadvantage of these products but is easily overcome. One strategy is to use butyl rubber, polyurethane caulk or 3M 5200 instead of silicone. The viewing window should be installed first with one of these products and then the Zavlar/Pond Coat is used to waterproof the inside and paint over the cured caulk. As an additional measure of security I recommend using a small amount of epoxy to first waterproof the area where the viewing window will be installed, such that epoxy layer bridges the seam between the zavlar and caulk. This way, in the event the zavlar separates from the caulk the epoxy provides an additional waterproof layer that will prevent leakage.
Liquid rubber summary
Pros: Highly flexible coating resists fractures even in poorly supported tanks. Easy to apply. Minimal smell.
Cons: Limited choice of colors (brown in the case of Zavlar/Pond Coat, bright blue in the case of Ames). Incompatibility with silicone slightly complicates window installation.