The Stovebolt Site Forums The Shop Area Paint and Body Shop Removing Rust with muriatic acid?

Does anyone have a good procedure for using muriatic acid to remove rust and not damage good metal? As far as I can tell you don't have to dilute the acid, but you have to make sure you keep a close eye on it. Then use Baking soda to neutralize the acid. Then use rubbing alcohol to clean the surface. The cleaned area must be primed and painted ASAP as the exposed metal will rust very quickly. Does this sound right? Is this a safe procedure? How affective is it?

Don't use Muriatic (diluted hydrochloric) on steel, you will get hydrogen embrittlement. Use phosphoric instead, it only attacks the rusty part of the metal and turns it black.

Just curious, do you know a cheap source for phosphoric acid?

Automotive suppliers charge a pretty penny for the stuff. I need to check chemical supply companies I'm thinking.

Also, do you know the normal dilution of the acid for rust removal?

phosphoric acid can be used fairly straight w/o damage, but can also be diluted w/ water - the more water the slower the action, and it can be used over and over til it collects too much junque

Bill

Phosphoric acid is an ingrediant in Coca-cola, so it can't be that difficult to obtain, especially since you don't need food-grade stuff. It is commonly found in the metal prep solutions that you use prior to painting. I'm not sure what the concentration is but the stuff I bought from Jasco seemed plenty strong. A paint supply or chemical store should be able to hook you up.

The phosphoric acid will cause a reaction that leaves a phosphate coating on the metal, gives the metal some protection.Commercial acid preps also contain other stuff supposedly to benefit the product. Considering what paint costs,the acid preps are inexpensive.

I can't seem to find phosphoric acid sold as 'metal prep' anywhere recently.

I've used the Jasco product and it's great stuff. I did not know it had additional ingredients which assist the process.

Jasco is about $14 a quart when I could find it, not outrageously priced. But often simple ingredients can be had at hugely lower price.

So I thought I'd check, I'm currently unemployed and pinching the pennies on supplies.

I buy phosphoric acid by the gallon at a local chemical supply. It's a 75% solution, which I'm told is the highest percentage available in WA.
I mix 1/2 cup of the 75% solution with 4 cups of water to use to convert rust . I'm using it to aid in removing rust from pitted sheet metal and for storing cleaned metal to prevent rust.
One gallon from chemical supply is $15.

A couple things come to mind. Never add water to acid to dilute it. You can add the acid to the water carefully, but the other way can lead to uncontrolled reaction and injury. A phosphoric product I use is called OSPHO runs around $20/gallon. Another product is EXTEND made by same mfg..
Another way to remove rust is electrolytic removal covered in threads here and in tech tips. After using that, you could then etch parts with acid, wash and wipe dry or use scotch pad and then paint.

Dupont makes several acid wash preps. If you're painting the vehicle,it may pay to use a commercial product,in fact all paint and preps should be from the same company.If a year later, the finish has a failure, the manufacturer may reinburse you for re painting supplies.
The first vehicle I painted was our 51 Chevy truck 8 years ago. I used all Dupont products including cleaners and acid wash.Now I'm no pro painter,the finish is nice but not perfect.8 years later,many hiway road trips, no winter use of course, the finish has no stone chips, no peeling, even where I painted under the fenders still looks good.

Thanks for the advice, I'll phone some chemical suppliersa tomorrow.

Just a few more questions!

I'm reworking my 4 doors and have ALL the glass and hardware out. I was planning to apply the acid with a spray bottle to treat the door inside surfaces.

- Do I need to remove all oil from the surfaces before the acid spray?

- Is a water rinse sufficient, or should I use a baking soda spray to neutralize first?

Thanks!

No luck with the chemical suppliers - only a few were willing to sell to individuals, and the price was high - $120 or so for a gallon of 85%.

Years ago, I bought a gallon of Nitric acid cheap. Since 9-11, I'm thinking chemical supply is more tightly regulated.

I DID find what I THINK is phosphoric acid at a decent price. The local tractor supply store (farming supplies) had a product that was $19.50 for a gallon. The sales person said it was the stuff that 'turned rust black', the ingredient list shows 'mineral acid and drying agent'. I assume it's dilute phosphoric acid with alcohol.

Perhaps the 5 gallon price for 85% would really turn out cheaper, it's just WAY more than I could ever use I'm sure. But I still don't know why I can't seem to find the stuff recently at auto parts suppliers.

This tractor shop also has paint at a really good price - $8.00 a quart for primer and enamel paint. Good enough for doors and floorboard, and I'll be able to get the hang of paint gun work on non-critical areas.

If you have oil on the surface, I would clean it with lacquer thinner. Water rinse is not necessary in my experience, nor is neutralizing.
Some suppliers of metal preparations reccommend rinseing with water, but you get almost immediate flash rust unless you can immediately dry the surface. If you have seams that water can get in, your asking for rust, IMO.
I spray phosphoric acid solution on surface and let it sit a few minutes then scub with a scotch brite pad and wipe up the dirty solution. Repeat as long as your removing rust. If the surface has stubbord rust pits, I use a stiff wire brush on a 4 1/2 inch grinder to scuff the pits and repeat the acid wash as many times as necessary to clean the metal.
If you let the solution puddle and dry on the surface you will get a black residue, but its easy to remove with another application of acid solution.
When I'm finished working on the metal for the day, I spray on the solution and immediately wipe it up so the surface is dry, which leaves it clean and protected from rusting as long as its kept dry.

Try an industial maintenance/cleaning supply. It's used around Puget Sound mixed with a cleaning agent to remove rust stains off painted surfaces on metal ships.
I'm on my third gallon of 75% solution.

Ahhh, it makes sense that marine supply is probably the answer for getting the stuff cheaply. I can understand the need with the corrosion that salt water creates.

I'm inland, and it seems that few people know of the stuff. Except for the tractor supply (and the expensive chemical suppliers), I only found one other place that carried it in 8 oz. continers! On the next trip to Florida I'll stock up.

Thanks for the advice!

It is getting more tightly regulated because of the havoc that phosphorous (specifically phosphates) cause in the environment- almost unstoppable algae blooms which choke out other life. It is a very good fertilizer.

Good call on the marine supply store, they have all the good stuff to deal with rust.

Here is a link for inexpensive phosphoric acid. http://www.prp-porstore.com/POR-15-Metal-Ready.html It seems pretty reasonable. Hope this post doesn't violate the rules. Tried to email jgart, but email sent back?

You might check with a local radiator repair shop to see where they buy acid.

I buy mine at Home Depot in the paint dept.