Cast iron tubs are usually finished with porcelain enamel. The process, which generally happens at the time of manufacture, requires high heat and special knowledge. It is not a task suited for do-it-yourselfers. However, you can finish your tub with an acrylic enamel for a fresh new look if you know the proper preparation process. Whether your tub is already coated with porcelain or still shows bare cast iron, it requires extensive preparation before it will accept a fresh enamel finish.
Cleaning - Scrub the tub, or the new enamel finish will not bond well. Ordinary cleansers will likely prove too weak to remove fine layers of invisible soap scum; use a trisodium phosphate cleaner.
Rinse the tub several times with wet rags. Be thorough; trisodium phosphate residue will also interfere with adhesion.
Dry the tub with towels or allow it to air-dry for two hours.
Prepping - Abrade the existing enamel finish to promote adhesion. A fresh enamel finish won't bond to slick surfaces. Sand the tub until it feels coarse.
Cover the metallic tub drain with painter's tape. Place canvas drop cloths over the floor.
Mask off all surrounding areas within 3 to 4 feet of the tub. Cover each of these areas with masking paper, taping it down where needed.
Priming and Enameling - Put on a respirator. Add 3 gallons of acrylic primer to a 5-gallon bucket. Lower the compressor's pump into the bucket. Spray a coat of acrylic primer onto the existing enamel finish. For best results, hold the spray gun 18 inches from the tub and keep the gun moving at all times. Let the acrylic base dry for two hours.
Wash out the bucket and fill it with 4 gallons of tap water. Run the clean water through the compressor unit.
Add the acrylic enamel topcoat just as you did the primer. Let the finish dry for at least four hours.