Polyurethane is a common modern wood finish that can be purchased with either an oil or a water base. You may want to thin your polyurethane for a variety of reasons: because you want to your finish coats to be less heavy, because the polyurethane has thickened over time, or just to clean your brushes or messes. Several substances can be used to thin polyurethane including turpentine, mineral spirits, paint thinner or, in some cases, water.
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Regardless of what you use to thin your polyurethane, incorporate the thinner in a careful manner.
Use a clean stirring stick (such as the type you can get free at most paint and hardware stores). Never shake the can as this incorporates air bubbles and will result in a less-than smooth finish. Instead, slowly mix the polyurethane with the stirring stick using a figure-eight motion. Make sure to scrape all thick material from the bottom of the can and continuing stirring until the polyurethane has a uniform texture.
Add thinner slowly, and in several batches. You don't want to make the mistake of overly thinning your finish.
If you are using a water-based polyurethane you can thin it with water. Make sure to check the instructions on the can before adding water to a polyurethane.
Mineral spirits have the advantage of being relatively odor-free and are therefore pleasant and easy to use. Mineral spirits are distilled from petroleum and are a mild solvent as well as a thinning medium.
A strong-smelling substance often used by artists, turpentine can be used effectively to thin polyurethane. Make sure to mix turpentine into your finish only in well-ventilated spaces or outdoors.
Paint thinners can be made from a variety of substances and have varying levels of odor and toxicity. Check that the paint thinner you buy is intended for use with polyurethane products.