Painting Cast Iron Valves
Question: We are a manufacturer of cast iron valves.
We are a manufacturer of cast iron valves. Having just joined the company in a QA capacity, I am alarmed at the poor quality of our paint finish and the sheer number of customer complaints with regards to this.
We use a two-pack epoxy on top of a primer. We receive our castings from the supplier in a primed condition and wash them down before topcoating. In some circumstances we have grit blasted the castings prior to topcoating; the appearance of these valves is much improved but I am unsure whether the paint finish is more durable with a better key.
Our painters are not time served painters and therefore are not 100% sure that what they are doing is right. What I am basically asking, is whether we should be grit blasting all castings to ensure that all contaminants are removed. Would this help us?
We find that by using our present process, stated above, we are creating an extremely high level of rework (close to 100%), especially during the winter months. We think this is due to outgassing of the cast iron. Is there another process that can help us achieve the desired surface finish, but give us no rework at a fraction of the cost? P. J.
Although you didn’t describe them, there are at least three possible causes for surface defects in iron castings, trapped moisture and solvent attack because of incompatible paints.
If moisture is the problem, it could be introduced during the winter months, at critical temperature and humidity conditions. Moisture will condense on surfaces when cold castings are brought indoors. It could also be introduced when you wash the castings. If there are any areas on the surface of the casting that are bare, the wash solution moisture will enter the pores of the casting. Air blow-off will only remove surface moisture. Instead, they must be heated to drive off the moisture from the pores. Painting over the open pores will trap the moisture in them.
If solvent is the problem, the solvent in the two-component epoxy paint is attacking and lifting the shop primer, which was applied at the foundry. To solve the problem make sure the shop primer applied at the foundry is compatible with epoxy paint you apply. If you want your paint finish to have an automotive-like appearance, have the foundry use a shop primer that is compatible with your two-component epoxy paint. Incidentally, the shop primer applied at the foundry should be a sandable primer-surfacer. Prepare the surface of the casting by using a body putty to fill any visible porosity and then sand to obtain a smooth finish. Use a high-pressure air blow-off to remove sanding dust before applying the epoxy topcoat.
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