The problem with true sand-blasting is it removes good metal along with the rust, after 35 or 45 years the frames may not have much to spare around the welds, etc. I was hesitant to sandblast though I did the steel wheels and parts of the 'A' frame with coal slag and was amazed how much good metal got cut away.
I ended up using a HS cut-off air tool with 1/4" shaft arbored 3M scotchbrite 2 or 3-inch sanding biscuit for lots of the rusted areas, and to strip the original undercoat asphalt-based paint easiest I used a 3" wire brush in the same cutoff tool. I also wore several wire wheel whiskers under my skin on my back for 5 or six days from laying on the ground to get at the frame. Magnet sweeper after every run! Anywhere inside the ladder frame that had no moisture damage got cleaned and painted with Derusto flat black for three coats; it seemed to melt into the original undercoat-style frame paint and bite really well.
Provided you get a second coat on at the correct point in its curing cycle to seal the rust-pit bubbles that were not wetted on the first coat, POR-15 is extremely good primer - and you only need to topcoat it where it might get exposed to UV in the sunlight; after almost two years still on blocks in backyard I see zero flaws in the still-bare frame.
POR-15 will not undercut, no rust tunneling under it. If you get the frame blasted clean POR-15 is still a superior choice, I thinned it down 5-7% with their thinner and got incredible coverage. Letting the bright metal frame get a flower of rust on it before POR-15 is actually better for the bond!
To answer your question - POR-15 is a near forever repair. If you have other products and got the hots to use them, go for it... get it done best as you can within available resources. But - if you want to spend a little more and get good re-selling point to flag those-in-the-know splurge for the POR-15...
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