Polyurethane is a relatively new finish that is gradually replacing traditional shellac, varnish and oil finishes.
Polyurethane dries to a hard film that wears well. Because it dries faster than varnish, it is less likely to collect dust particles as it cures, so it needs less sanding and rubbing.
When do-it-yourselfers attempt to buy polyurethane they are often confused by the terminology on the can; “urethane,” “polyurethane,” “spar urethane” and “liquid plastic” are commonly used terms. These terms are used by marketing departments in an effort to position their products. The terms are similar; they refer to products that produce the same type of finish.
Most polyurethanes are oil-modified formulas with a vegetable oil (usually linseed, soya or safflower) base that has been treated with polyhydric alcohol and disocyanate.
The chemical reaction between the oil base and other ingredients takes place in a large (at least 1,000 gallons or more) closed stainless steel container.
To avoid discoloration, the air in the container is evacuated and replaced with carbon dioxide, then heat is applied. The result is a concentrated resin that must be thinned with solvents and blended with additives to produce a workable product.
While some manufacturers tout their product as being clear and non-yellowing, all polyurethanes will yellow to some degree over time. It is the oil base that will eventually turn yellow.
Linseed oil will turn the deepest shade, while safflower oil will remain relatively light. Safflower oil polyurethanes are therefore a better choice for furniture made of light-colored wood. Even though it darkens more, linseed oil-based polyurethane produces a slightly tougher film than the other oil-based formulas and is a better choice for outdoor applications.
High-gloss polyurethanes generally have a tougher film than the satin polyurethanes.
Applying polyurethane is not difficult, but care must be taken to ensure the bare wood surface is free of wax, oil and dust.
