Paint Peeling Off Cast Iron

Question:

We are a manufacturer of cast iron components on which we paint. Before painting we do vibratory finishing. The chemical we use in the machine is alkaline based and the media we use is alumina-based ceramic media. The alkaline chemicals are used to prevent rusting after vibratory finishing. After they are dry, the components are painted. Our problem is the paint is peeling off the cast iron components. Can you tell us why we are having this problem? Can you help us to solve the problem? R. M.

Answer:

As you know, cast iron surfaces are porous and your problem is related to that porosity. Before painting, any surface must be free from oily soils, particulates, corrosion and moisture. Alkaline cleansers can be used to remove oily soils; wiping, brushing and air blasts can be used to remove particulates; chemical pretreatments can be used to remove corrosion and heating can be used to remove trapped moisture.

My guess is that moisture trapped in the pores is causing the problem, and it is introduced when you vibratory-finish the castings. The finishing solution will not be removed from the pores of the castings by the air blow-off, which will only remove surface moisture. Instead, the castings must be heated to drive the moisture from the pores. Painting over the pores will trap the moisture in them and cause subsequent paint peeling. The dried residue from the anti-rust chemicals may also contribute to paint adhesion problems and it should probably be rinsed off before painting.

Before painting, I suggest the following procedure: 1) Rinse the castings, to remove the alkaline anti-rust solution residue. 2) Blow off the surface moisture using compressed air. 3) Heat the castings to drive off any moisture trapped in the pores. 4) Allow the components to cool and then paint them.