POR-15 Metal prep?

[eswanson]

From my understanding the metal prep is to do some neutralizing but mostly it helps with etching non-rusty areas. If you are coating an older piece with no new metal then simply applying the POR-15 should be sufficient to encapsulate and prevent deterioration from rust for longer than we are likely to see the end of. I've seen examples without the use of metal prep that are a couple decades old and still look good.