Removing Rust from Manifold
Forum rules
If you need help logging in, or have question about how something works, use the Support forum located here Support Forum
Complete set of Forum Rules Forum Rules
If you need help logging in, or have question about how something works, use the Support forum located here Support Forum
Complete set of Forum Rules Forum Rules
-
Topic author - Posts: 103
- Joined: Thu Jul 23, 2020 9:28 pm
- First Name: Bruce
- Last Name: Brakke
- Location: Ames, Iowa
- MTFCA Number: 31192
Removing Rust from Manifold
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.
-
- Posts: 3281
- Joined: Fri Jan 04, 2019 9:37 pm
- First Name: Steve
- Last Name: Jelf
- Location: Parkerfield, Kansas
- MTFCA Number: 16175
- MTFCI Number: 14758
- Board Member Since: 2007
- Contact:
Re: Removing Rust from Manifold
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.
The inevitable often happens.
1915 Runabout
1923 Touring
1915 Runabout
1923 Touring
-
- Posts: 592
- Joined: Sun Jan 06, 2019 12:23 pm
- First Name: Jeff
- Last Name: Humble
- Location: Charlevoix, Mi
- Board Member Since: 2006
Re: Removing Rust from Manifold
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.
Last edited by Humblej on Wed Oct 07, 2020 7:05 am, edited 1 time in total.
-
- Posts: 46
- Joined: Sun Jan 06, 2019 1:57 pm
- First Name: john
- Last Name: hardiman
- Location: stoughton, ma
Re: Removing Rust from Manifold
Make small electrolysis tank..... only attacks rust.
-
- Posts: 1442
- Joined: Sun Jan 06, 2019 8:00 pm
- First Name: John
- Last Name: Kuehn
- Location: Texas
- MTFCA Number: 28924
Re: Removing Rust from Manifold
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!
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!
-
- Posts: 2792
- Joined: Sat Jan 05, 2019 1:57 pm
- First Name: Mark
- Last Name: Gregush
- Location: Portland Or
- Board Member Since: 1999
Re: Removing Rust from Manifold
Outside inside or both? Intake exhaust or both? Outside, wire wheel or sandblast. Inside I sandblast to knock any crud loose.
I know the voices aren't real but damn they have some good ideas!
1925 Cut down pickup
1920 Dodge touring
1948 Ford F2 pickup

1925 Cut down pickup
1920 Dodge touring
1948 Ford F2 pickup
-
- Posts: 3281
- Joined: Fri Jan 04, 2019 9:37 pm
- First Name: Steve
- Last Name: Jelf
- Location: Parkerfield, Kansas
- MTFCA Number: 16175
- MTFCI Number: 14758
- Board Member Since: 2007
- Contact:
Re: Removing Rust from Manifold
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.
The inevitable often happens.
1915 Runabout
1923 Touring
1915 Runabout
1923 Touring
-
- Posts: 34
- Joined: Wed Feb 06, 2019 11:52 am
- First Name: Reuben
- Last Name: Kugel
- Location: Pewaukee WI
- MTFCA Number: 49871
- MTFCI Number: 24323
Re: Removing Rust from Manifold
I have used a product called Evaporust with get success. Only attacks the rust and neutralizes the rust so it does not start again.
1915 Touring
1926 RAJO Speedster
1926 RAJO Speedster
-
- Posts: 588
- Joined: Sun Jan 06, 2019 10:55 pm
- First Name: James
- Last Name: Patrick
- Location: Bartow, FL
- MTFCA Number: 50126
Re: Removing Rust from Manifold
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
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
-
- Posts: 290
- Joined: Sun Jan 06, 2019 12:42 pm
- First Name: David
- Last Name: Dewey
- Location: Oroville, CA
- MTFCI Number: 19936
- Board Member Since: 1999
Re: Removing Rust from Manifold
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.
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.
T'ake care,
David Dewey
David Dewey