As Kotzy said, "Make sure that both surfaces are clean, as well as the bolt threads. Lay a straight edge over the surface of the thermostat housing to make sure it isn't warped." If the housing is slightly warped as has scratches you can use a metal file to even out the surface, if no file is available you can place a piece of 150/220 grit sandpaper on a flat surface and slide the housing back and forth until the warpage or imperfections are mostly gone. As far as gasket sealer goes, I like to use Permatex No.2. Use a piece of cardboard or paper plate and apply a moderate coat to one side and even out the layer with your finger, flip gasket over and apply a coat to the other side in same manner. I lift the gasket off the cardboard surface and inspect the first side for any gaps in coverage, if so use the sealant still on your finger to fill any gaps in coverage. I then squeeze some sealant on my index finger and thumb and I coat the entire threads of the housing bolts. Allow sealant about 10 minutes to get tacky, insert thermostat into recess, apply gasket, put housing in place, insert bolts finger tight, and torque to specs. It works for me, good luck.