Phosphoric acid use on cast iron?

[Alan Riley]

I recall reading somewhere a while back that phosphoric acid should not be applied to cast iron as a part of paint preparation as you might do with steel but I'm not sure where I saw it. Checked the labels on two products containing phosphoric acid that I've got out in the tractor shed - DuPont Metal Prep and Jasco Prep and Prime - and neither says anything about not using it on cast iron. Any thoughts? Experiences?

[Buzzard Wing]

I know you should not use it on aluminum. So keep it away from the serial number plate.

Other than that I can't say.

[Slappy]

I use it on cast iron all the time with good results. It's hard to get all of the rust out of cast iron. A wire brush is good to a point, but has a tendency to burnish the iron fooling you into thinking you got the underlying rust. Sandblasting is better at getting the rust, but the sand gets everywhere it's not supposed to be. Acid is a good choice I think, it converts the rust and etches the iron which helps the paint adhere better. If you put the acid on haevy with a brush, or treat with multiple coats, it may leave a glaze that won't hold paint - to avoid this problem just hit those spots with your wire brush before painting.

[Alan Riley]

Thanks for the info, Buzzard Wing and Slappy. I will go ahead and use it on the cast iron as well as the steel.