Cast iron Claw-foot Bathtubs: How to Clean, Remove Rust & Stains, Restore, Re-surface, Recoat

A. Hi B. The black oxide process you are describing is still widely done on things today. Industrial gears, sprockets, couplings and chain are black oxided. Although the corrosion resistance is quite limited, it's an attractive finish that doesn't change the dimensions of the objects like plating or painting does. The same process is widely done on firearms, although people then tend to call it "blueing" rather than "black oxiding". When it's applied to a rough surface it comes out matte black, but when done on a highly polished surface it's comes out closer to a jewel-like navy blue. These are finished surfaces which are not subsequently painted. It might have been used on some of the claw feet of cast iron bathtubs, but I'm not familiar with that. A gun shop can do this process for you.

When you intend to paint a surface, you would instead use a phosphoric acid formulation called naval jelly or rust converter [on eBay or Amazon] to convert red rust to that Fe₃O₄ black rust; this is something which any home owner or hobbyist can easily do, and you can certainly do to the OUTSIDE of your tub if there is any rust. Good luck.

Regards,

A. The thing is, Neil, that the original finish was probably porcelain enamel, more like melted glass than paint -- a ceramic frit (powdered glass) is melted and fused at extremely high temperature onto the cast iron. You can't do that, sorry; so you're restricted to paint (which, sorry, just isn't nearly as good) or finding a porcelain enameling shop to do it for you.

Second best is to hire a professional tub refinisher who can do a good job with premium quality enamel paints. Third best is to purchase specialty touch up enamel [on eBay or Amazon] made for that purpose and do it yourself, recognizing that this will not approach the durability of a factory-applied molten coating. Good luck.

A. Hi Robin. Brass colored paint and brass plating (real metal) are not the same thing, so you'll need to decide whether you can settle for the paint or must have real brass plating.

Although it is not completely impossible for a hobbyist to do any metal plating at home, brass plating on cast iron would involve sandblasting the cast iron, acid pickling it, nickel plating it (both for shiny appearance and because you can't plate brass directly on cast iron), then brass plating (this is cyanide based), then several well-applied coats of brass lacquer or a 2K automotive clearcoat. In short, if you want real brass plating, realistically you'll need to be able to remove the claw feet and send them to a plating shop.

If you can settle for brass colored paint, get any old paint off as best you can by sandblasting if available to you, or by sandpaper and paint stripper if blasting isn't possible. Then apply a phosphoric acid based naval jelly or rust converter [on eBay or Amazon] . Cast iron is porous and you'll probably have rust stain problems if you don't do this. Then do a brass color outdoor paint, brushed or sprayed as you prefer. Topping off with a couple of coats of clear acrylic spray may give a somewhat more metallic look. Best of luck.

Regards,

A. Hi, Skadi. For the outside, you can deal with the rust with naval jelly [on eBay or Amazon] before repainting. If the inside is porcelain, you can't really repair this yourself as the coating is more like a melted glass than a paint. But you can probably touch up chips with touch up enamel [on eBay or Amazon] , and if there are rust stains, lots of people on topic 8229 swore by hydrogen peroxide [on eBay or Amazon] . Good luck.

Regards,

A. Rachel:

I have had this done on two cast iron sinks. First you will have to find a powdercoater that is willing to take on the project. We first tried to sandblast the surface of the porcelain to roughen it for the coating to adhere to. The first sink took the powdercoating well,but the finish was orange peeled because the sandblasting was done too hard and left the surface of the porcelain too rough. The second sink had some small cracks in the porcelain that allowed out gassing and created large bubbles in the finish. We ended up sandblasting the second sink down to the bare casting,sanding the surface smooth,filling and sanding pits and craters in the casting with a compatible powdercoat filler and then recoated the sink. Overall I am pleased with the results even though after several attempts the second sink still had minor out gassing (pinholes). Sanding the bare casting makes for a nice finish but is very dirty and time consmming.

I hope this helps.

A. As you have just purchased it I'm assuming you have not installed it. Which is good, muriatic acid [affil links] will eat through the rust, the porcelain glaze will keep the rest of the metal from being eaten, do this in a well ventilated area as it will give off lots of fumes.

Ed. note: although the ventilation is to protect your lungs, any stainless steel or bright metal in the area can get rusted by the fumes too.

Q. I too have clawfoot bath tub with rust looking stains. Would like to know if the treatment worked on Ray W's bathtub.

A. Hi, Ray; hi, Janie.

Although Marc knows his stuff, and his answer probably works, please see letter 8229 first ... because dozens of people swore by the simplicity, economy and effectiveness of hydrogen peroxide for removing rust stains from bathtubs, and I'd hate to see muriatic acid used where a much safer & simpler approach can work. Good luck!

Regards,

A. I had the same dilemma as Ray W in Texas. I tried everything, then I tried powdered Oxygen Cleaner (generic), I mixed 2 scoops in 4 cups of hot water, plugged the tub and let it sit for 15 min., did this twice and it worked with very little scrubbing. Did the trick.

Hi Jen. You need paint stripper designed to take off whatever it is that you put on :-)

There are some coatings that can only be removed with Aircraft Stripper (methylene chloride) -- but it is truly noxious stuff you can only use with gloves, goggles, and really good ventilation. Hopefully a milder stripper like Citristrip can do it.

Regards,

A. Hi Gopa. "You can't make a silk purse out of a sow's ear", and you make thick, glossy, wear-resistant, rock hard, porcelain enamel by spraying a molten glass-like frit at extremely high temperatures; you can't make porcelain enamel from body putty and spray paint :-(

It's not going to be completely satisfactory, but you can only do what you can do. There are 'adhesion promoters' that will hopefully keep the paint from blistering off of the porcelain. Good luck.

Regards,

A. Hi Sue. I'm pretty sure you could thicken hydrogen peroxide with fumed silica (the powder/fluff you see for that purpose in epoxy filler kits). But whether any household product like cornstarch or flour or baking soda can be used without consuming most of the oxidizing power of the peroxide, sorry, I don't know.

Regards,

A. Try wallpaper glue (Carboxymethyl Cellulose based); it must work ... Hope it helps, and good luck!