Which type of undercoating is best?

Which type of undercoating is best depends on whether you mean house painting or car rust protection, and your climate and goals. For house painting, best undercoating is primer suited to surface, such as oil based or shellac based primer for bare wood with knots to block tannins, PVA drywall primer for bare drywall, and bonding primer for glossy surfaces, as these seal and provide adhesion. Combined primer undercoat products from reputable brands like Zinsser, Benjamin Moore, or Sherwin Williams are best for general use. For car underbody rust protection, there are several types, rubberized undercoating is thick hard coating that provides good physical protection against stone chips and lasts 2 to 5 years but can crack and trap moisture if applied over rust. Polyurethane or epoxy based hard coating is more durable and best for new cars wanting long term protection lasting 5 to 10 years. Oil based or wax based undercoating like Krown, Fluid Film, NH Oil, or Waxoyl is best for old cars and rust belt because it creeps into seams, displaces moisture, does not trap rust, but needs annual reapplication and is best for rust prevention. So best for new car long term is polyurethane, best for old car and salty roads is oil based annual spray, best for house is surface specific primer.