Adding a clear finish to a set of cornhole boards is common practice. While most people will use a water based polycrylic, many will use an oil based polyurethane. Polyurethane has many advantages to polycrylic, with one major disadvantage. Polyurethane will often give a slightly yellow tinge to the surface where is it applied. If you have used white as part of your artwork, the color will be slightly yellowed.
Commonly Used Products
Rust-Oleum® Wood Care Interior Ultimate Polyurethane
From pharaoh - Rust-Oleum® Wood Care Interior Ultimate Polyurethane is what many here use. As good or better than polycrylic.(an arguable point) Available at Lowes. About $32.00. If there is a Sherwin Williams paint store close by, you might consider doing the sign up thing with them. They'll send you coupons for 30-40% off that is unadvertised to the public as well as notifications when they have their usual sales for 50% off paints.
From the Minwax Website
Minwax® Fast-Drying Polyurethane is a clear, hard finish that dries fast to protect and beautify interior wood surfaces such as furniture, cabinets, moldings, and doors. Minwax® Fast-Drying Polyurethane is among the most durable of protective coatings. Minwax® Fast-Drying Polyurethane’s long-lasting protective finish makes it ideal for use on both finished and unfinished wood.
- Surface must be dry and free of old finishes in poor condition, paint, wax, grease, polish, dirt or other foreign matter.
- Sand to obtain a smooth uniform surface. Remove all dust with a cloth dampened with mineral spirits.
- Stir Minwax® Fast-Drying Polyurethane before and during use to eliminate settling on the bottom of the can. Stir in such a manner as to rotate the product from the bottom to the top of the can. NEVER SHAKE.
- Apply a THIN coat of Minwax® Fast-Drying Polyurethane using a high-quality natural or foam brush.
- Let dry 4-6 hours. Then lightly sand entire surface with fine sandpaper (220 grit) to ensure an even finish and proper adhesion. Remove all dust.
- Apply second coat. If third coat is desired, repeat step 5 before application.
- After final coat, allow 24 hours before light use.
Information copied from http://www.minwax.com/products/oil_based_clear_protective_finishes/fast_drying_polyurethane.html