I am posting this for those members that don't know about this stuff. I learned about it on this forum and I am absolutely amazed each time I use it. What does it do? Removes rust and preps the metal for painting (gives metal a grip for the primer to grab onto).
I am stripping my car for primer. On the roof I have some pitting and rust, I assume someone parked it for a long period of time under a tree and allowed wet leaves to build up on it, causing the pitting and rust. I don't really want to sand it since some are very deep (no inside rust) and I think it would thin out the metal just to clean out scratches. So first I stripped the paint using aircraft remover, neutralized it. Then I cleaned half the roof with CLR and a synthetic steel wool pad. After rinsing that off.. this is the first coat of Phosphoric acid on it. It will probably take a couple coats, followed by a wire brush. But I don't think I will need to sand/remove any of the metal to have it rust free.
The first 2 pictures are of my truck's tool box, rusted as all heck. I laid the paper-towel I was spreading the phosphoric acid with, down on the tool box for no more than 10 minutes. It's the before and after picture.. no scrubbing or anything.. just lifted it up. Imagine if I worked on it!
The other two pictures show the 2 halves of my Mustang's roof, one worked on as stated above, the other is only paint stripped.
If you are thinking of painting or want to remove rust, look into phosphoric acid. I picked it up at a home improvement store in the household paint isle. I'm sure there are products that work better, but for a metal prep tool, P.A. is wonderful on the rust! (Rinse fully before painting!)

 

Yup, great stuff If you really want to go at it, muratic acid works even better, but it's extremely corrosive and really needs to be completely neutralized after use. I wouldn't use it on body panels, though. It's commonly used to clean rusty gas tanks before sealing them.

 

Phosphoric acid isn't NEARLY as hard on the skin if you get it on you as Muratic acid either. Muratic acid is nasty stuff.

 

Use a heat gun to warm the metal before hand. It'll work better. Buy a 1.00 plastic spray container and use that to spray it on. Keep it wet (not running wet) and allow to sit for 20 to 25 minutes. Spray throughout that period as needed to keep the surface wet. Rinse and if needed repeat.

Thats a roof I'd hit with 2 coats of por-15 for sure and epoxy primer afterwards.

 

It's not as ad as you think... The pink is the last layer over the original metal, only the thin scratches have rust. The car has original black paint (under two other layer of paint and primer), then a layer of white and then grey primer... grey is a bisch to remove... but the milky-pink is what coated the metal originally, doesn't all come off with the aircraft stripper... but will come off with the synthetic steel wool and elbow grease, almost like a plastic coating (?) maybe a sealer? I thought it might be red-oxide primer mixing with the white primer? But, It feels more like a plastic when sanding. Water and steel wool will take it off, revealing bright, shiny metal (with the rusted scratches).
Even when I sand all the layers off instead of using aircraft remover (in all areas of the car).. it has this milky-pink layer over the metal.. Looks like rust in the pictures because of the shade and small picture, but it is actually only in the lines/scratches that are rust. Looks great except for those!
As for the spray bottle? Absolutely! Full strength in a spray bottle is the way to go, just watch for over-spray on unprotected surfaces. That's what I use the shop towels for, to stop any over-spray. I'll have to try the heat gun, never heard that one!

 

After rinsing the bare metal doesn't it flash rust. How do you prevent that

 

I never had any real problem with that when I did my car. For example I took the insides of the front fenders down to metal and the final step in that process was to rinse with water. I dried them and they were fine for some days thereafter until I primed and painted.
Working on the back quarters I had metal exposed for quite a long time in the areas near the body filler where my sanding of filler would remove the nearby paint. If I left that a week or two I might get a little rusting, which was no problem to remove.
Some of that might depend on the humidity, time of the year, etc. I was generally in an air conditioned garage in a part of the country where humidity was not a big issue.

 

It is very important to remove all of it, no white haze after applying it. Then you use water to remove/neutralize it. Get it dry immediately, using a towel and it helps to follow with compressed air. There shouldn't really be any rust remaining, there may be flash/dust rust from the water. When everything is ready to prime.. You'll take care of that.
Mix 3 parts water to 1 part phosphoric acid, diluted it will work wonders on this surface dust/rust. Use a cloth dampened with the mixture, not wet with it! and sort of "polish" the area. Follow that with a damp (water rag) to neutralize any remaining and dry it off immediately. Then take a clean/dry rag dampened with acetone and work the entire area, dry behind if you like, but it will dry immediately. That will help neutralize any remaining acid and also remove any oils or grease and make your area clean for primer. Prime immediately. If you wait any number of days, you should do these steps again. you should have bare metal with teeth to grip and no oils a perfect surface for primer.
Take a look at the rag you used the diluted mixture, it will be covered with dirt and red rust, even if your metal looked clean! I always do this step before painting anything, it' amazing how much rust is on clean metal.
It is very important not to paint over dried PA. You can tell it is present by a white haze on the surface. Paint will eventually peel off if you leave any. So remove it with regular or diluted PA and maybe a scotchbrite pad to wet it again, Remove with lots of water and dry repeatedly until it is all removed. (easier to remove diluted)
Give it a try, you'll swear by it!

 

Ok, here is a pic of the "after". This is after the entire roof has been treated one time. Some people might be happy with that result, but I think it should take another "treatment" since there is metal pitting down the center that might hide rust. This time I am going to try a different product... some POR-15 Metal Ready.
I just picked up a starter set of POR-15 and noticed that the Metal Ready is a mixture of Phosphoric Acid and Zinc Phosphate. I've been curious about the zinc coating it is supposed to leave behind to help prime the metal. I know the PA works, so I am going to give it a shot instead of just the PA.

 

would using coke get the same effect?