Phosphoric Acid Rust Removal

[tbgallant]

Looking at using Behr Concrete etch (45% phosphoric acid) to treat surface rust on my Miata restortation project.

From my research/reading it works quite well. I'm just wondering about the application/removal process. Lots of folks say rinse with water.. not sure I want the mess/work and potential rerusting from doing this.

Others have just wiped/brushed acetone (or wax/grease remover) afterwords. Some say to do this before the acid dries, other say do it after (white residue forms that you want to remove). I'm going to be welding after the treatment so I dont' want something that will mess up my welds.

Ospho is a similar product and the directions for it say you don't have to rinse. I'm not covering any of these areas with high-tech auto paints. Plan to either use spray bombs or POR15. I'm not sure if the Behr stuff has any other chemicals that if not rinsed will cause problems?

Any folks have experiance with this?

Thanks

Tim

 

[krooser]

If you use POR-15 you won't have to pre-treat with anything other than a wire brush for any flaky rust.

I occasionally use Milk Stone Remover for lite rust... it's also phosporus acid based. Never had a problem rinsing with water after using it.

 

[Bull]

Ospho is the better choice, IMO. Easy to use, and the no-rinse is key for me.

As for not having to use acid under POR 15, I have not found that to be true. About two months ago, I sanded some rust in the radiator support on my Firebird, then cleaned the area and applied POR 15 black. The rust is ALREADY coming through pretty noticeably. Wish I had used some Ospho!

 

[Treeman]

bgallant, I don't have experience with the Behr product, but I have been internet researching pre-paint metal prep products for months and cannot come to any conclusions.

Seems like there are rust removers, rust converters, etchers, phosphate coaters, rusty metal primers, and products that say they do one or many of the above.

A google search of rust converters will turn up a few real research papers (U.S. Navy, etc.) that say they don't work as claimed.

My most recent search of "metal prep" (the old name of a dupont automotive product) turns up lots of controversy in the auto paint forums. Some "claim" that paint peels off certain metal preps. Some of these guys are talking about this product with raves:

http://www.picklex20.com/ But, how do we decipher all this marketing/internet hype?

Some products are supposed to be water rinsed, others not. Even though the Behr product is Phosphoric acid based (like many others) it MIGHT contain wetters or other chemicals not intended for metal or paint covering. Just my opinion....I really don't know. IMO, I would err on the cautious side and use a product labeled for the intended use.

Let us know if you learn more.

 

[tbgallant]

I too have read about Picklex20. Sounds like a great product but just too darn pricey for me. The phosphoric acid approach really seems like the way to go.

I don't see how phosphoric acid converting the rust doesn't work (per Navy research). Folks have defenitely been doing it with great results (on cars..) for a lot of years.

I can see how the paint guys are worry about different things not 'cooperating' with each other. I'm not really that worried about such things, I can always keep an eye on the paint adhesion and reapply (it'll just be rattle can most likely).

I'm leaning towards just cleaning either after or before drying with wax and grease remover. I'm just wondering how long I need to leave it on (guess I can experiment) if I clean it off before it dries.

I'll let you know if I learn more.

Tim

 

[benjamming]

We have used phosphoric acid based products at work to remove rust & scale from carbon steel. It works fine, just like sulfuric acid works fine. However, I don't have any experience with this product. What parts on the car will you be using this on? What material substrate? What is future plans for these parts? There is almost always a water rinse (purified water actually such as DI or RO) in the chemical treatment arena where I work.

 

[tbgallant]

I will be using it on my floor pans (the parts that are still solid just have surface rust, heavy rust is being cut out and replaced with new metal) and have been considering it on my suspension control arms.

The floor pans will have it brushed on (and not sure the procedure I have decided for removal). The control arms I'm planning on soaking in a bucket of diluted mixture and then rinsed via garden hose. I'm not sure of the phosphoric acid strength needed for this, for a 24 hr soak. Any suggestions?

What strengths of PA are you used to using and what is the procedure?

Thanks

Tim