Sanding floor pans Sucks

POR 15 holds up very well if it is applied right, over rust, follow the manufactures instructions.
My jeep frame still looks great after 5 years after using POR15, I cheaped out on the axles and springs and used Rustolem guess what small rust spots are appearing.
So you get what you pay for.

In you case with the floor pan, just brush on the Rustolem and move on with the next project.