My bathtub is cast iron and 62 years old. The previous owner was
elderly and wasn't able to clean very well. It has quite a bit of
calcium, lime, soap scum and rust deposits. From reading this group
and various other sources I know I need to clean it with acid I'm just
not sure which would be better - phophoric or oxalic acid (aka wood
bleach). I know oxalic is poisonous and phosphoric is little easier to
deal with but I want the best results possible. Refinishers do an acid
cleaning before they refinish a tub. Anyone know what they use? I
would appreciate any info you may have regarding acid use for cleaning
a tub with these kinds of stains.
Also, after cleaning does it make any sense to use a polishing
compound to buff out the tub? I'd like to restore whatever luster or
life it has left in it and I don't want to resurface. I was thinking
of a buffing compound such as what is used on cars after they are
newly painted. I believe this has a componenet similar to a very very
fine grit wet/dry sand paper. Aren't tub paints similar to what are
or used to be used on cars?
Any thoughts would be greatly appreciated.
Please e-mail direct to firstname.lastname@example.org
Thanks very much.
Phosphoric acid is the ingredient in naval jelly, and would be good for
Oxalic acid is a bleacher and would be good for organic stains.
They would use a really strong acid to etch the surface, so the new
finish sticks. You wouldn't want that.
Hydrochloric acid, available at the hardware store as muriatic acid, in
quarts or gallon jugs, does a good job of cleaning calcium, lime and
soap scum. It can be diluted down (do as you ought to, add acid to
water) to make your own cleaning solution(s).
What "organic stains" are you referring to. The advantage of oxalic
acid is that it converts insoluble iron salts (stains) into soluble
oxalic acid salts of iron, oxalates. Iron is about as inorganic as it gets.
Phosphoric acid doesn't do a very good job of dissolving lime scale.
Citric acid does much better. If you can find someplace that sells it in
bulk, it's not very expensive -- I think I paid $5 for a pound of it
recently. You can use it with Dawn Power-something dish washing soap and
dissolve the lime and soap scum at the same time.
Citric acid will be a lot easier on your lungs than any more powerful
alternatives I know of. Sprinkle a little on, rub it in with a wet rag or
brush, add a little dish detergent, rub a little more, and let it soak in
for a while. Scrub with a brush. Rinse. Repeat.
I'd consider it more of a scale preventative, intended for long term
contact (ie: a dehumidifier).
For scale/rust removal, I'm partial to "Zep" brand "Calcium Lime and Rust"
remover, available from HD. Prolly around $5 US per gallon. A small
fraction of the cost of CLR, and somewhat stronger.
Active ingredient is hydrochloric acid, but a lower concentration (more
in my comfort zone) than hardware store style "muriatic acid" - last time
I opened one of those, we had to evacuate the house. [container was
prolly >15 years old, and the amount of HCL gas that escaped was
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
For soap scum, try a razor blade scraper first (at least on the flat
areas) to get the major crud off. Then scrub with 3M pad and full
strength Dawn dish detergent to get the rest of the soap scum. Then use
CLR (calcium lime rust) on the lime. If the glaze is eroded away and
the iron exposed, go easy in those spots.
True. Oxalic acid is poisonous but you'll find it in spinach and
coffee, to mention a couple of foods that have reasonable amounts. Just
be careful working with it. Remember the DOSE is the poison.
and phosphoric is little easier to
For removing calcium build up I'd favor phosphoric acid (which can also
be a nasty acid). Gloves and eye protection are in order.
I'd imagine that they use hydrofluoric acid to etch the porcelain. Of
the three acids this is highest on my list of dangerous acids. You can
get some nasty burns with HF.
Be very careful that you don't abrade the porcelain.
I'd like to restore whatever luster or
Cast iron isn't painted (normally). It is porcelanized which means it is
heated and small glass particles are applied; they fuse (melt) and coat the
None of the chemicals you mention will attack a porcelain surface so once
you get the crud off it should be in good shape. If you *do* need to use an
abrasive, try it on a small, inconspicuous place first because many are hard
enough to scratch the porcelain surface...you don't want that.
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