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Derusting a frame.


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Draker's right.


If your doing a rebuild you have to get rid of the rust.


Rust reformers, converters, encapsulaters, naval jelly just don't get the deep down rust and it will come back.


I've tried phosphoric acid, muriatic acid, and most commercially available rust preventatives.


None have done a satisfactory job.


Sandblast the shit out of it.


Use the best primer you can afford under the lowest possible humidity conditions.


Go from there.

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Some places still use an acid bath to dip car bodies and frames, but you never get it out of the seams, and its almost impossible to seal all internal surfaces.  Sandblasting works best.  An inexpensive shop might charge $100 to blast a frame clean, or up to $500 to blast and seal in epoxy primer.  Strip it 100% first, or you'll be finding sand everywhere you don't want it.  Inside the diff, steering box, everywhere.  Its unavoidable unless you full disassemble.  


By thoroughly sanding it first, you can keep the frame "wet" with a 50% phosphoric acid mix (like Milkstone remover found at TSC) to dissolve the remainder of the rust in the pits. Its ineffective if its not warm (70+ degrees).  


I agree to skip the rust encapsulators.  They are no more effective than just painting over it, which is essentially what you'd be doing.  

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Mechanical removal followed by metal etching with phosphoric acid should get the job done.


Depending on time/budget/scope of rust, I use a wire wheel, a needle scaler  (you can get a cheap Harbor Freight one here - http://www.harborfreight.com/air-needle-scaler-1108.html), sanding discs, and as a last resort I go to blasting.


After you get the junk off, spray on some metal etch - https://www.amazon.com/Krud-Kutter-Metal-Clean-Gallon/dp/B005FMZ3NW  and then pressure was it off. Dry it quickly to avoid flash rust.


Then use a good etching primer - https://www.semproducts.com/aerospace-refinish/self-etching-primer and top coat with engine or chassis paint. I like to use engine paint because of the high ceramic content which has a high resistance to chemicals.

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Some places still use an acid bath to dip car bodies and frames, but you never get it out of the seams, and its almost impossible to seal all internal surfaces.  


Many vehicles were never sealed on the internal surfaces to begin with.. but still valid point. 

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do the hard-to-reach spots with sandblaster but from experience i recommend doing the rest with a grinder. Use POL-15 on the frame. It adheres to grinded rust very well. I can tell you that after 2 years it holds up fine. The exceptions are in between leaf springs where the brush couldnt reach and suspension parts where they move or capped with rubber.

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Sebpv, i found the same thing on my leafsprings.. once installed and with weight on it they exposed some inner edges the were missed.... went around last week chasing down spots like that....

I used por15 on the frame ... actually I used it on everthing.... use a top coat of you use por15 in an area that will see direct sunlight... doesn't degrade but it will slightly discolor

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The rust between your leaf springs actually prevents them from working well.  The leaves are supposed to slide on each other.  Rust acts like grit between them.  If you want a decent ride, take those leaves apart and clean them up.  Places like Speedway Motors carry a slick liner you can place between leaves so they slide properly on each other and provide the much desired and deserved suspension travel.  

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The 620 frame is 'boxed' but the metal is just folded and welded on a seam half way up on the inside of the truck rail where it rots away unseen. My rear spring perch rotted away and separated from the truck. (I was wondering  what that rattle was) I trimmed the perch metal and had a length of 1/4" steel plate bent into a U shape that fit up over the frame from underneath. Two weds along the top and weld the perch to the underside.

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