Hot Rod Mag , did a write up about all the rust remover's a while back.
one thing that they did was make a recirculating system with a homemade trough and plastic sheeting to direct it back to the trough with a pump in the bottom .
the fluid was then allowed to spray out of a home made pvc spray bar for a few hours and the rust was removed clean.
If i had to do it i would try this approach.
also i have real world experience with P0R -15
most of the problems people have is they don't read directions.
por-15 will peel off off clean unsanded or sand blasted metal if not treated with their version of ospho called Metal Ready ,
if you follow directions it is like powder coating hard!I have it on many projects. por also adheres to rusty metal not clean metal or untreated with metal ready. IT practically self crawls to cling to surface rust.
that said if the rails are cleaned to bare metal clean i would chose to use Dupont Variprime self etch primer sprayed in with garden sprayer .
Cj