A small number of two-part epoxy coatings have the unique feature of being able to be applied underwater. Common applications include coating piers, pilings, pools, tanks, etc. Most applications using these epoxies however are not applied underwater, but rather to damp or water saturated surfaces. Examples would include concrete waste water tanks (drained, but certainly saturated with water), and sweating pipes, vessels, and loading docks. In many areas of the globe, humidity levels are often well above 75% nearly all of the time. With such high moisture levels, most dry surface paints will have a much reduced surface bond which can result in premature blistering or coating failure. Simply put, moisture, in whatever form, is a primary reason for coating problems. Underwater (i.e. wet surface) epoxies avoid this area of concern. These epoxies can, of course, be used on dry surfaces much like any other coating. Besides those already mentioned, other places you are likely to find underwater epoxies include the inside and outside of ships, inside manholes, on cooling towers, piers and pilings, and tanks, pits and sumps of all kinds, as well as on floors with vapor transmission problems.
Underwater epoxies work by displacing the water between itself and the object surface. Air, like water, is considered a fluid too, albeit a much less dense one. Underwater coatings work the same way regular paints work on dry surfaces only instead of displacing the air, they displace the water. Few coatings work underwater, not because of the problem of displacing the water, but rather because of the affect of the water on the coating itself. The water can either act as a solvent or actually react with one or more parts of the coating and thereby interfering with the normal drying or curing of the coating. Underwater epoxy components are completely unaffected by moisture or water reacting normally whether water is present or not.
All underwater epoxies are not the same! Some manufactured outside the USA, contain a dangerous chemical known as MDA. Others require heating to approximately 120 degrees F for proper application. Finally, a number of so-called underwater epoxies, typically some of the first underwater coatings, work more like bubble gum and must be quite literally pressed into the surface in order to stick, like bubble gum stuck to the bottom of a desktop