There are several types of durable, moisture-resistant finishes ...
Oil Modified Polyurethane - is generally the most common surface finish. It is durable and moisture-resistant, and generally the easiest to apply. This type finish tends to amber slightly as it ages.
Water-Based Finishes (Urethane and/or Acrylic combinations) - These are clear, durable, non-yellowing and are non-flammable at time of application. They generally have advantages similar to Oil Modified Urethanes without the odor of mineral spirits, and they dry much faster.
Moisture Cure Urethanes - are harder and more moisture resistant than the other surface finishes and are generally available only in gloss. They cure by absorbing minute quantities of moisture from the air, which causes them to dry and harden. Relative humidity is critical to the curing process. Follow manufacturer's directions. These finishes are extremely difficult to apply properly and are best left to the professional.
Swedish Finishes (Acid Cure Urethanes) - are also durable finishes, generally harder than polyurethanes. They are clear, fast drying and resist yellowing. Type of undercoat, working time, number of coats, and other factors are all critical and make application difficult. These finishes should be applied ONLY by the highly skilled.