sandblaster or wire brush + rust neutralizer?

I'm a fan of Por15 over sandblasted metal.
but... the problem you might run into is that Por15 is so hard and shiny when it dries that there won't be much "tooth" for the fiberglass to bond to. I found it difficult to sand it was so hard.
After the body flexes a few times I would expect the glass to start popping loose from the floor plans.
I believe Por has their own "Rust Patch kit" that you might check into.