[Flathead]

You should use a product that does not harden, and will penetrate seams and rusted areas. Fluid Film would do that, also the waxy products. Detailed application is very important as well. Eastwood has some good "rustproofing" products. Plain old undercoating is not good for this. An oldtimers solution was to give the whole underside a good heavy spray coat of drain oil, then ride a dusty road to cover the drain oil. Whatever you decide, be sure you clean off any salt first. Salty roads are bad for vehicles.