Warehouses, contractors and large shippers are good sources for used pallets. They are very often thrown out; so chances are someone will be happy to save on the disposal fees by giving them to you. Make sure the wood is not contaminated with any toxic or degradable materials. The wood should be intact but otherwise imperfect and varying in patina.
When the first coat is dry, lightly sand the entire surface to remove any lifted wood grain. Apply the second coat heavier and work it into any cracks and crevices. The clear coat will accentuate the colors and textures on the slats such as these original mill lines and paint.