What Paint To Use For painting Under The Tub?

I've used SEM Rust Mort and the directions state;

APPLICATION:
BRUSHING:
Apply multiple coats, keeping the area wet with Rust Mort until all rust is converted or removed. The longer the
surface is kept wet, the better the final result. After completion, remove any excess or non-converted Rust Mort
by rinsing with water and a brush. Make certain all surfaces are completely dry before proceeding with body filler
or primer per manufacturer’s recommendations. Failure to remove excess material can result in loss of adhesion of
subsequent top coats. Limit exposure after drying the surface to eliminate flash rust.

https://semproducts.com/public/content/techsheets/sem_tds_rust-mort.pdf

So yeah, you have to keep these products wet, at least the stuff I'm familiar with. It's that or completely submerge it. Rust Mort works well, but it's strong enough to etch into concrete if left unchecked. You have to neutralize Rust Mort.

I’d never argue with the Jeep Guru ! He is called a wizard for a reason , man knows his shit to the tea.
 
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There will be a lot of areas where you can't remove the rust completely. Look at something like the KBS Coatings conversion primer to slow down how much the rust will continue. Their top coat is pretty stellar in the satin for frames. Not fond of the price but it looks pretty good when you are done.
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That KBS top coat is looking phenomenal.
 
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Gotta agree with mrblaine! Good prep work is a lot of work which you will remember! The extra cost for quality paint, will be forgotten, except for the longer lasting job.

I just finished doing a bed on 2011 F250, that’s lived life in upstate NY where drivers rely on an incredible amount of salt in winter. Different than your application though, since I took bed off, I was able to sandblast it and frame relatively easily, though sandblasting sucks. 3 coats of Southern Polyurethanes (SYP) epoxy primer. Tried Raptor liner for 1st time. Was very impressed and just sprayed the undercarriage of bed with it also after primer. If you are able to spray and get rid of your rust first, SYP epoxy primer is an excellent product and does not need a topcoat. They host an excellent forum, that many pros frequent and willingly give advice on their experience. SYP only recommends Ospho ( and no others!) under their primer ONLY if you cannot remove rust. They do provide several cautions on properly and neutralizing. I never had to use it, but think their advice is very good. Check their website. The owner of SYP was the creator of the paint, highly helpful to all customers and long following of pros using his paint. Lots of threads on doing frames on their site.

Here’s a shot of SYP epoxy primer on my 2011 frame

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Stay AWAY from people that tell you to booger it up with Fluid Film

It's hard to tell exactly what "booger it up with Fluid Film" is supposed to mean but here's something to keep in mind:
Its one thing to say “I disagree” and heres the basis
Its another thing entirely to come off like an asshat mouthy person with your demeanor
 
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It isn't and I'm weary of spending the same amount of time and effort and then using crap spray paint when all that work is put into something.
Is this the stuff you used? How did you apply it roll on/ paint gun / aerosol can ? Not sure how much I would need to do the entire uncarraige of my Lj.
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I prefer ospho for removing rust on many things, but it chalked up on me when I applied it Vegas on my 96 Chevy dually frame that had been neglected on the coast by the PO.

I ended up extensively wire wheeling it to ensure my GF would hate me. IIRC, I then scrubbed using dawn soap and water, wiped it down with aircraft cleaner, and taped off the body before hitting it with aerosol primer and paint.

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Like stated earlier prep work is everything. Back when I was dealing with heavy scaled rust I would use a powerful needlescaler (maybe not on thin sheet metal), then wire brush with angle grinder, Eastwood rust converter, then Eastwood rust encapsulator. Then two coats of rustoleum rusty metal primer, sanding between the coats, and two coats of rustoleum topcoat, sanding between the coats.

Fluid Film is amazing stuff, but you are only supposed to use it on a vehicle you will likely never need/want to paint again. Pressure washing might remove 75% of it, but the only way to completely remove it is with towels.