Place the poster board onto a flat surface. If possible, tape the corners down with painter's tape. This will help prevent the paper from rolling if you get it slightly too wet.
Apply a small amount of polyurethane to the top of a paintbrush if you want the polyurethane to have a textured look. If you want a smoother look, apply a small amount of polyurethane to the top of a paint sponge.
Place the tip of the paintbrush in the upper corner and apply a downward stroke to the surface of the poster board. Go from the top to the bottom of the project instead of across the face. Make sure you use a single stroke instead of brushing in multiple strokes. The polyurethane will wet the paint as you go across it. If you avoid multiple strokes you will not disturb the paint.
Wet the paintbrush again and place it next to the first stroke. Apply a second stroke next to the first stroke. Make sure the second stroke butts up to the first stroke but keep the overlap as minimal as possible. Repeat the process for each additional stroke until you cover the entire project. Allow at least 24 hours' drying time.
Add a second coat to the project if designed. Follow the same process as with the first stroke.