Pressboard is much more porous than woods like pine, oak and cedar and requires different preparation. If you try to paint Pressboard the same way as regular paint, you will get premature peeling or warped wood.
1. Sand rough saw cuts and variations in the wood for a smooth finish. Be gentle on the edges of pressboard, they can break off easily.
2. Wipe down the entire painting surface to remove all sanding debris, dust and particulate.
3. Apply two coats of primer. Two coats are needed to make a smooth enough surface for the final coat(s) of paint. Older latex primers had problems with the glue, but current ones are fine.
4. Paint a finishing paint over the desired area until you get the correct color. This should only take one coat, unless the color is drastically different from the primer.