Cast Iron Painting Problem

Q. I found your e-mail address on PF Online, with a very detailed answer to a question from a cast iron valve manufacturer. So I’m going ahead with a question for the expert.

We are just a trading company. We bought some cast iron bathtubs from Northern China in February 2006. They were sent to a customer in Vietnam. Those products are standing bathtubs. The inside is covered with porcelain enamel and the outside is painted. Upon receipt, the customer found all of them to have paint problems—many bubbles formed under the paint layer.

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Of course, the weather conditions can’t be more different (very cold and very dry in Northern China, very hot and very humid in Vietnam). But it also seems the factory hasn’t mastered the coating technology at all. Do you have any suggestions for them to improve their process? All they seem to be doing now is applying two layers of spray-paint. G. B.

A. There are three possible causes for paint blistering on your cast iron bath tubs: oily soils on the surface, trapped moisture in the casting and incompatibility of the paint layers. The cast iron tubs must be cleaned to remove oily soils before painting. They should be heated to drive the moisture from the pores. The paints used must be compatible.

If the tubs are porcelain enameled and then stored outside in the cold before painting, they can pick up moisture after being brought inside. If the moisture is trapped in the pores and painted over, it will cause blistering of the paint. If they are handled extensively, they will pick up finger grease and other oily soils, which will also cause blistering of the paint film. If the paints are not compatible, blistering can occur. I can only guess the manufacturing process but I can tell you what it should be.

The inside and outside surfaces of the tubs must be cleaned before enameling. The cleaning process will remove oily soils. They should be handled as little as possible after cleaning. The relatively high processing temperature of porcelain enameling will drive the moisture from the pores. The tubs should be painted as soon as they are cool enough, immediately after enameling. Finally, they must use high quality paints, such as a catalyzed epoxy primer and a polyurethane topcoat, to withstand the abuse a free-standing bath tub would get.