Removing Rust from Manifold

[BLB27]

I plan to soak the manifold in "Krud Kutter-Must for Rust" to remove rust inside. The product is phosphoric acid, will it damage the cast iron? I have been using it on steel and it really works.

[Steve Jelf]

It will be fine. Just don't leave it in for weeks or months. I use the stuff for metal prep on steel and cast iron. It delays new rust until you can get the paint on. Do NOT let any get on your clothes. You can get clear high temp finish for exhaust manifolds.

[Humblej]

Yes phosphoric acid can damage cast iron. The damage is not immediate, but the longer you soak it the more noticeable it will be. It will soften the cast iron like paint remover softens paint, you will be able to scrape the outer surface off the part with a wire brush or your fingernail. Casting marks and threads may disappear. Great stuff for plain steel, but cast iron will soften, and hardened steel like spring steel will disappear in phosphoric acid. I agree with Steve that it is a great treatment to finish the derusting process for steel to protect against immediate future rust. If done right it leaves a golden hue to the metal that resembles aluminum anodizing. Prevents rusting from the moisture in the air and allows time for painting.

[hah]

Make small electrolysis tank..... only attacks rust.

[John kuehn]

Paint and body stores sell Metal Prep or or something similar in gallon jugs to use when giving auto body parts or entire bodies a wash before final prime and paint. It has phosphorous acid in it to remove any oils or residue and to give good adhesion for the applied coatings.

It’s good to use for small parts derusting since it’s a weakened solution of phosphorus acid. Since it’s a weak mix and if you forget you left you parts in a container it won’t emulsify your parts as quickly!

[Mark Gregush]

Outside inside or both? Intake exhaust or both? Outside, wire wheel or sandblast. Inside I sandblast to knock any crud loose.

[Steve Jelf]

Paint and body stores sell Metal Prep...

Yes, they do. I bought it for many years. Then I discovered that I can buy a gallon jug of phosphoric acid and mix it 50/50 with water for a fraction of the cost.

[rnkugel]

I have used a product called Evaporust with get success. Only attacks the rust and neutralizes the rust so it does not start again.

[jiminbartow]

If, when you use acid on cast iron to remove rust and the area where you applied the acid turns white, with tiny bubbles, the acid is attacking the cast iron and will dissolve it completely, in a short time, so it is essential that you remove and neutralize the acid immediately with baking soda mixed in water.

Cast iron is not like steel and does not respond to acid like steel does. Cast iron is porous and has a lot more impurities than steel. Acid can get into the pores of cast iron and destroy the it from the inside out as the acid eats its’ way deeper and deeper into the cast iron as the pores below the surface are opened up.

Use acid only on steel. Not cast iron or aluminum. Jim Patrick

[david_dewey]

Cast Iron is rather porous, and will hold the acid in the pores, leading to its degradation.
As for Metal-Prep and other products, read about your paint first, as some work better on plain untreated steel/metal (DP Epoxy primers are one example). I would use glass-beading on the manifold with careful cleaning afterwards.