general finishes white undercoat, 1 gallon general finishes general finishes white undercoat is similar to general finishes sanding sealer with the addition of white colorant to provide a primer undercoat for white/off-white wood finishes. the high solids facilitate a fast film build and sand easily. use as a basecoat for pigmented acrylic finishes. it dries tack-free in 10-20 minutes. particularly effective on mdf.