What to use for undercoating?
Not really… It's more like factory rust proofing with body color overspray.
But I may be misinterpreting your "properly" with "correctly".
Anyway…….. Eastwood is best for separating a fool and his money. (They are good for those hard to find items).
SEM, M3, Truck bed liner……. All local options at a fraction of Eastwoods cost.