OK; Rust-proofing/Undercoating?

I like Rustoleum products and I like the roll-on Dupli-color bed liner (with chunks of rubber in it). I think these products are designed to remain flexible for a long time. The Dupli-color bed liner is designed to go on over paint.

The factory asphalt-based undercoating on my '68 GTX was cracked and falling off in places by the mid 1970s. I think very low temperatures make the stuff brittle when it ages, allowing moisture underneath when small cracks appear.