My brother-in-law just finished building me a custom rack for my 75 f250. As it is now, it is unpainted - raw steel. I will need to paint it or apply some sort of coating on it to protect it from rust.

Some things I have considered:
-Rustoleum Hammered - would brush on - not sure about durability.
-Al's DIY bed liner - Used it before putting my drop in bed liner - seems durable - a bit pricey.
-PlastiDip - would seal nicely - but likely tear easy too?
-Rustoleum textured spray paint - durability?
-Line-x or Rhino Liner - Ideal but likely going to be expensive.
-Professional powder coating - not sure about durability.

Any recommendations on this?

Here is a shot of the rack resting in place (it actually moves about 6" closer to the back of the cab where it will be bolted to the rails.)

 

powder coat or line x it. everything else you mentioned will just end up failing in the long run and youll be repainting it again. I had my bumper line x'd about 5 years ago and it still looks great and I dont have to worry about rust or chipping paint. It cost $160 but its worth paying the extra money and doing it right with quality stuff.

 

Because you have a flat truck I think some paint-able undercoating with some flat black paint over the top of it would do it.

Love the FLAT black paint on your truck!

 

You could use a two-part epoxy primer. 2 coats will protect it from any rust, and dry to a black satin type finish. I get mine from this manufacturer; http://www.southernpolyurethanes.com/

 

If you prime it with epoxy primer it should be top coated, especially if its in sunlight like your rack. Ive never used SPI paint but, from my experience epoxy primers are not UV resistant and should be top coated.

 

Also if you decide to get Line x it will fade to a flat black and match your truck. As long as you dont have them spray with the coating that keeps a gloss.

 

The SPI epoxy primer is very UV resistant, unlike other epoxy primers. Barry has added plenty of extra UV protection to his formula. Check it out here if you don't believe me. SPI User Forums

 

I didnt know that Barry mixed his epoxies to be more UV resistant. I ve been wanting to use their stuff for a while mainly because they sell it for such a good price. Ive heard a lot of good about their clear too, have you ever used it? I actually want to use their primers and clear when I paint my truck.

 

I just recently started doing the body work on my truck (as in this year). I am using all SPI primers right now. In the future I will probably be using their bed-liner. I am also going to check into their black base-coat, and their clear. I have never done anything like this before, so I am not sure if I will be doing the painting myself or taking it to a body shop. I have been teaching myself how to do the body work and spraying the primer. I am on the SPI forum also, and like this forum it is full of very knowledgeable people. Here are a few links to my project/build, and the SPI website, and forum.
https://www.ford-trucks.com/forums/9...build-log.html
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SPI User Forums

 

POR15 is supposed to be really good. UV's degrade it though, so you'd have to topcoat it.

POR15, Inc. - Stop Rust Permanently - Repair Gas & Fuel Tanks

 

But it doesn't look like rusty metal. It looks like a fresh build that needs to be protected from rusting.

 

I pretty much taught myself how to paint/bodywork a few years ago. I was able to practice on friends cars. I just dont have the time to do my own yet. My advice if you do it yourself is focus on prep, prep, prep. Most of your time is spent prepping and dont cut corners because your finish will show. Also make sure you spend the money and get a good gun for clear, like an Iwata.

 

You can use POR15 on new metal, you just need to prep it differently.