Prime & Paint underside questions

Roger we make our own product for floorboards trunks and under body that has performed well in Michigan for us. We use Rustoleum Rusty metal primer. This stuff will stick to any metal surface as long as there is no loose rust or other material. If you want to make it insulate like "Lizard Skin" you can add the microscopic glass spheres to the Rustoleum primer. Mix it thick enough to brush on two coats allowing it to dry between coats. The spheres are available from several places. We use Hy-Tech Sales.com. We use an airless sprayer but a brush works. We undercoat the bottoms with 3M and use sound deadner on the floor pans over the glass spheres and primer. Seems to hold up well here in the Winter Water Wonderland. Dan