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Apparently, POR15 doesn't offer UV protection:
https://www.por15.com/POR-15-FAQS_ep_61.html
"WILL THE SUN DESTROY MY POR-15 COATING IF I DON'T TOPCOAT IT?
If the surface is exposed to the sun for an extended amount of time, the UV rays will eventually break down the POR-15 and cause it to fail. That?s why we recommend you topcoat it."
It is true, plus it needs to be applied to BARE metal to be effective. Applying over coated surfaces is a waste.
 

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After driving my rig through a couple of more winters I realized that merely applying Ospho and Fluid Film is not enough. Red rust kept coming back in some spots.
So, this advice is based on what I’ve learned from MED over the years, and recently on this project: corrosion protection is a matter of layers: each layer protects the one beneath, and helps the one on top of it to stick.

1) remove as much red rust as possible (wire brush the parts in place, or better yet, remove as many parts as possible and sand blast and re-coat separately before re-installation)
2) apply phosphate wash (Sherwin Williams sells Ospho) with a cheap, disposable garden sprayer (note, phosphate will attack zinc plating, so keep away from any fasteners which are still silver colored). The phosphate wash will convert any remaining, microscopic iron-oxide into an inert iron-phosphate. But note it is still reactive and will not last without some kind of covering protection against oxygen and water.
3) note that paint cannot stick to dirty surfaces, so clean, clean, clean with rubbing alcohol, brake cleaner, etc. until the rag comes back clean. If you have FF already on the frame, clean that off with oven cleaner until the surface is ready to accept paint.
4) paint all surfaces with a good quality top coat (something like POR15, for instance, it is epoxy based (I think) which includes UV protection as well as 02 protection)
5) after the top coat of paint is dry, go over that with body wax, unfortunately this isn’t readily available at most auto parts stores as a consumer product, so I buy Amsoil Metal Protectant, which is sort of like “Cosmoline”, and seems very similar to the body wax Toyota uses on line. It will dry to a semi-hard, non-tacky surface. Use a long straw with a 360deg spray nozzle to tread inside of frame rails, suspension arms, etc.
6) once dry, go over that with a lightweight liquid rust protectant like Fluid Film, it is lanolin based, so is OK to get on anything (on rubber is OK), but don’t get on brakes (obviously), or tires (slippery). FF is great stuff, but it does tend to rinse off, so should be re-applied each fall

Note that FF can be applied with a bulk sprayer, from a gallon can, the first time, then after that first big application each year a single rattle can and some long straws, and about 1 hour of time is enough to re-do the whole underside. For it I don’t even bother to mask or do any cleanup, simply sweep of a comfortable part of the driveway, slide around on a piece of cardboard, wear dirty clothes and then let the weather clean off any overspray/drips from the driveway when done.

Any time anything like this is done, go back and make SURE all of the drain holes are still clear (not clogged with undercoating) to prevent pooled water causing corrosion after all.

Whenever any fastener is removed, use some anti-seize on the threads before re-installation (note, when using it, which is a lubricant, reduce torque -40%, keep a record and re-check at that new torque value after a few 1000 miles just to be sure).

Hope this helps,
Norm
Apparently, POR15 doesn't offer UV protection:
https://www.por15.com/POR-15-FAQS_ep_61.html
"WILL THE SUN DESTROY MY POR-15 COATING IF I DON'T TOPCOAT IT?
If the surface is exposed to the sun for an extended amount of time, the UV rays will eventually break down the POR-15 and cause it to fail. That?s why we recommend you topcoat it."
Top coat is a good idea but why are we talking about UV protection unless you plan to park your FJ upside down. :huh:

POR 15 works very well in the winter by the way. I did a snow plow blade and mechanism with it about 5 years ago and it's still holding up well. I used the metal prep for areas with exposed rusty metal. No chips or scratches to date.
 

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note that the black factory coating which all of the underbody parts were coated with is also not UV stable, but it holds up quite well because it isn't exposed to the sun

the problem the factory coating suffers from is lack of coating of the welds, the rust starts at them first
 
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