Wire BrushUsing a wire brush to remove the paint will usually only wear your arm out and provide you a mediocre surface for your primer coat. Obviously, we don’t recommend using a wire brush.
SandpaperAn electric sander is a decent option depending on how many layers of paint you are dealing with. Remember, you will be creating a lot of dust when sanding so a well-ventilated area, dust mask, and safety glasses are required. You really should think safety - the paint you are taking off will likely contain lead, as most old paints had lead in them. Start with a coarse to a medium grade of sandpaper and, when you get to the cast iron layer, work your way down to a fine grit.
Chemical Paint RemoversChemical paint removers are another option. If standard paint removers don’t work well, you can go to an auto parts store and ask for a stronger Aircraft Stripper. Chemical strippers are convenient, but exercise care when using them – they usually have strong chemical odors and harmful fumes. If you use chemicals, make certain to follow the safety precautions.
SandblastingSandblasting is the most thorough way of removing paint. If you decide to have your tub sandblasted, make certain to tape off the edges of the roll rim, drain hole(s) and any faucet holes with a strong duct tape. This will help prevent damage to the porcelain during the sandblasting process. Again, you really need to think safety here if you decide to do the sandblasting on your own. Remember, you will be blasting chips of paint from the surface of the tub with a sand frit, and you don’t want to get that stuff in your eyes.
Once you clean the surface, you need to get a primer coat down as soon as possible. This is particularly true with a sandblasted tub. Exposed cast iron can begin to show signs of surface rust within hours of exposure.
When choosing paints, find a primer that adheres well to metal and a top coat that stands up well to moisture. When it doubt, look up the paint manufacturer online or ask the staff at the paint desk at your local hardware store.