Problems With Enamel Cast Iron Bathtub Finish

If the finish on your cast iron tub is failing, the most likely culprit is inadequate or improper preparation at the time of application. There are two types of cast iron tubs: those coated with porcelain and those left bare. Each requires special preparation before it will accept a new finish. Learn these important procedures before you attempt to repair the failing finish on your tub, or recurrence will prove inevitable.

Bare Tubs
Bare cast iron tubs aren't suited for new finishes. Before they'll accept an epoxy or enamel finish coating, they require priming. Unfortunately, ordinary primers don't bond to cast iron. A metal etching primer is required for these types of tubs. If the finish on your cast iron tub is failing, it's probably because you or someone else applied the finish directly over top of the bare cast iron; or it could be because you used the wrong type of primer.

Porcelain Coatings
Many cast iron tubs are coated with porcelain. Like cast iron, porcelain is ill-suited for new finishes. Before the porcelain coating will accept a new finish, it requires abrasion. Professionals sand porcelain to create a rougher surface, better suited for primer adhesion. Once the porcelain-coated tub is abraded, it will accept an acrylic primer. If the finished porcelain on your cast iron tub is failing, it's likely because you or someone else forgot to abrade the porcelain or failed to apply a base primer.

Solutions
If the existing finish on your cast iron tub is failing, you must refinish it. Scrape loose particles from the tub using a metal putty knife. Abrade the existing finish with sandpaper, or the new finish won't stick. If areas of bare cast iron are showing, prime them with an etching primer. Once all these steps are taken, you may apply a fresh enamel or epoxy finish.

Considerations
Sometimes, improper cleaning can lead to finish failure. Dirt, oil and soap scum will prevent adequate adhesion. Be sure to degrease the tub before applying the fresh finish.