What Is a Polyurethane Finish?

Polyurethane finishes provide a hard, durable gloss for crafts and model-making and can be applied to almost any surface. There are several types of polyurethane finishes and paints, and several ways to apply them. The size of the craft project and its use will determine the best type of finish and method of application. You can choose from simple spray-can finishes to extremely durable and high-gloss two-part finishes.

Identification

Polyurethane finishes are the most commonly used finishes for crafts. The finish will be very thick, hard and glossy, much like an automobile finish. They are also frequently used for boats and marine applications. When a clear finish is applied over wood, it has the appearance of a smooth, plastic shell.

Features

Polyurethane is a plastic-based resin, available in satin, semi-gloss and high-gloss finishes. It can be applied to most types of plastic, fiberglass, wood, metal and most fabrics. The most valuable feature of polyurethane finishes, apart from a flawless, glossy finish, is resistance to water and chemicals, including gasoline. This makes it ideal for model-making. It is high in solids, making it slow to dry, but creating a thick, durable film.

Types

Polyurethane is available as a clear topcoat or in many colors. Most hobby shops sell polyurethane finishes in spray cans, already mixed and ready to use. For larger projects, a two-part finish can be mixed up. This requires some care, because if the finish and the drying catalyst are mixed incorrectly, the resultant finish could bubble and crack with use. For painting items like model airplanes, a more flexible and slightly thinner polyurethane formulated specifically for its flexibility is a good choice. Polyurethane finishes are usually applied with a spray gun or spray can, although some formulas can be brushed on.

Considerations

As with most finishes, the best method of application is in several thin coats instead of one or two heavy coats. Since polyurethanes are hard and glossy, they will show every surface imperfection, so starting with a very clean, smooth surface is essential, as is sanding between each coat. Follow label directions for application methods and drying time.

Warning

Use adequate ventilation when working with polyurethane finishes because the solvent fumes are very strong. If proper ventilation is impractical, use a canister-style respirator. Don't rely on a paper or fabric mask because these will not filter the fumes. Never use these finishes near exposed flame or furnaces because they are extremely flammable. Wear proper eye protection, and avoid prolonged skin contact.