I live in ohio where salt is bad. The undercarriage is already pretty bad with rust as it is a 91 and never had an undercoat. I was thinking of lifting the truck up and sanding / wire brushing the rust off over the weekend. Im just wondering how the process works? is it get rid of rust->
use rust converter primer-> then use something like fluid film? does a rust inhibitor get put into there? I don't want to use the rubberized undercoating because iv heard horror stories of it making rust accelerate.

 

Anything that traps water against the metal will accelerate corrosion. The key is to make a continuous unbroken layer, beginning at the TOP (which is the trick) so that water doesn't flow in at the top & collect as it runs down inside. Once the frame is rusty, it's not likely that you'll ever stop it, unless you strip the entire frame (which I've done a few times, but only to frames withOUT much rust). This is the Bronco I drive now:

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This is a '75 Bronco I built for a friend that we Herculined (which I don't recommend):

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This is a '93 Bronco I'm building now on a '96 frame:

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If you still want to try it, I recommend you begin with a home pressure washer, a set (or 2) of truck ramps, and SEVERAL cycles of soap, pressure-wash, rinse, let it dry, inspect with a flashlight, & repeat the next day you feel like getting filthy.

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I just want to slow the process of rust down not stop it. Is there anything that can be done without pulling the frame off. All I have Is a lift and tools

 

Trying to halfass it will likely cause it to rust worse, for the reasons I said above. If you have a lift, it should be easy to lift the body off the frame. In any case, you'll have to start with the pressure washer, so try that & post pics of what it looks like after a few cycles.

 

Just spray over everything. once you cut off the oxygen, the rust will not continue. just like a fire, cut off the air and it will self extinguish.