Acrylic high build primer surfacers fill well and are fast drying for easy sandability. These primers should be thinned at a 1:1 ratio with acrylic lacquer thinner or urethane reducer. Spray at 40 to 50 PSI in double wet coats, allowing each double coat to flash before the next is applied. Sanding may begin 30 minutes after the final coat. If using over large areas of steel or aluminum or any bare metal, Kirker suggests pre-treating the substrate with a metal conditioner such as a self etching primer.