I second the POR15.
Then top coat with the bed liner for durability. The top coat on the POR I believe is due to its UV issues not durability. POR will fade in the sun after only a few days of exposure.
The POR will stop all the active rust and is quite durable by itself but will also provided a great clean surface for the bedliner material to adhear to. The bed liner alone may allow the rust to continue under it and blister.
If the spreader was new factory paint the bedliner should be fine alone but since the metal is rusted, even if you sand blast it I believe your results will be better with the combination of the two.