Yellow glue: It sizes up as a pre-prime sealer
When you need to prime and paint a porous material, you can save considerable time—and primer—by first sealing the parts with yellow glue thinned with water, known as glue size. The glue reduces the absorption of the primer and avoids the need for a second coat. It also sands much easier than primer.
To prepare the glue size, mix three parts of water with seven parts of glue. Apply the size with a foam brush, avoiding contact areas where you need to glue parts together. Let the glue dry overnight. Then sand the glue smooth, working from 150 to 220 grit. Remove the dust, and you're ready to prime and paint.