Phosphoric Acid to prevent rust

Greetings,

I got a gallon of 85% food grade phosphoric acid for rust treatment on some old ford fenders and hood off of a 640 - the sheet metal is well beat up (before I got the tractor). This is a work tractor and it will be very loved helping around the farm; my only goal is to preserve it. I wanted to test out using the phosphoric acid before I used it on a restored item so I figured this would be a great chance.

Now, Looking over the internet, I have heard many ways to use the phosphoric acid, some say use it straight up, neat without cutting it. Others say to reduce it 4 to 1 with water. Still others say to dillute it to 12 to 1. Some say to let it dry for 24 hours, others say to rinse it off in 30 minutes.

Everything on the internet is always correct, right? Well, I have found that the people on this group are very good and in the past I have gotten lots of great advice; you have not steered me wrong. So, what to do you reccomend?

I am going to prime it with Rustoleum gray primer K7781, and use the rustoleum farm & implement paint in ford gray to top coat it. I will be spraying it with a HLPV gun from harbor freight. Ackward spots may get brushed.

Like anything there are probably multiple right answers. Here's what I do:

Use a spray bottle and apply it, keeping the metal wet. It will bubble and foam, it's doing it's thing. I try and keep it in the shade so it doesn't dry prematurely. When it's done bubbling, or about 30 minutes I rinse it. I then let it air dry. (Remember I said multiple ways? I have also allowed it to dry without rinsing it. I find if I'm not rinsing it, I like to water it down slightly, seems to leave a better finish) Then I slap some primer on and start my body work.

Do you cut it or leave it at 85% in the spray bottle?

Do an experiment.

Put some in a 5 gallon bucket and soak a rusty item like a log chain an axe head. See how well it works at different %.

Put some on your rusty fender. To prevent it from evaporating, cover the wet fender with plastic wrap you use in the kitchen..

Phosphoric acid will not eat metal. It converts rust back to iron. Rust is iron oxide. Phosphoric acid removes the oxide, oxygen, which causes the bubbles..It should turn the metal black.

I would use it straight. It can't hurt the metal. You do an experiment and post your results with and without plastic.

Phosphoric acid is the right stuff. I used it in the early stages of my roadster refurb in 2015-2018. As well as many projects before and since. I started off using Rust Mort, manufactured by SEM. SEM specializes in products for the auto body industry. It is a top notch product but very expensive though it goes a long way. More recently I have been using "Delimer" that I get from the manufacturer that I work for. Before using the delimer you want to have cleaned off all the rust that you can with your cup type twisted wire wheel in your grinder. I follow the Rust Mort instructions. I use it STRAIGHT. Being watery thin it takes very little. I brush it on with one of the small brushes that you use for soldering paste. I keep it wet for a good 15 minutes agitating frequently with the application brush. The chemical needs a little mechanical help to move the loosened rust aside. It needs aggressive rinsing. I toss small parts into a bucket of water, larger parts need pressure washing. Excess that is not rinsed off will show up as white residue once dried. You don't want very much under your primer.

If its a part I may not use for years I let PH dry when I need the part I scuff it with a scotch brite pad. If I am going to paint the part soon I flush it off with water before it has a chance to dry. I mostly use it to get a clean oil free surface @ 100% this works especially well on new metal. I have cut PH when I am running low it cleans just as well as 100%.

If I am treating rust I use Extend are Loctite SF 754 apply and let dry. I scuff when I paint the part, Youtube it.

Remember neither are epoxy primer friendly scuff are not epoxy will not bond well to them. The only way I have seen to get either off is to apply another coat to dissolve it.

For metal prep I am going to paint soon cut PH 50% flush with water I like to get a dusty residue on the surface not a hard polymer surface. The best metal prep is from a automotive paint store its exspensive... : (

For heavy rust Extend scuff a little before paint.

If using any body filler it needs a clean rust free bare metal surface to bond to.

Using your cheap paint I doubt you will have an issue with adhesion BTDT if an exspensive... automotive paint YES.

To be clear it does not PREVENT rust. As soon as it is finished doing its thing, the metal immediately starts rusting again.

The whole idea behind diluting it is the more concentrated it is, the more hazardous it is to handle.

Check what concentration you have to start with, then adjust to what you need.

For rust neutralization, you should be able to find the concentrations that are used in commercial and consumer products by looking up the products online.

If rinsing, I go straight in the spray bottle.