I'm on the same page as Alvin,although I didn't need my chassis blasted (it didn't need it),there was more caked on dust and grease than rust with a fair bit of the original factory finish still left on it.
I went down the POR-15 route.
I've basically painted the whole chassis,the underside and inside of the cab and under all four fenders

I know the basic concept of POR-15 is 'paint over rust' but so far it's holding out just fine and the dirt washes off easily.

Just as well as i'm in cattle country,it's not just mud....

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.