Inner Fenders, which undercoat or liner?

[69SCSS]

I am going to have my inner fenders blasted and was planning on painting the tops, but wasn't sure what I wanted to do with the bottoms. I've seen different things on the board, undercoat, bed liner, POR15, etc. Any thoughts? I'd like to make sure they don't rust out, figured it would be a good idea to do the tops of the inner fenders at the usual rust spots at the core support. What did you guys do to your inner fenders?