cleaning masonry / limestone

Quoted text begins.I'm writing in hopes of receiving feedback about methods that are safe
and effective for cleaning limestone masonry construction.
End of quote.

Bob, I'd like to comment on your situation in addition to JVC & Will's advice.

Will wrote:

Quoted text begins.Least invasive methods first, I say.
End of quote.

I agree.

A solution of Lux soap and water, lightly scrubbed with a soft broom is my first
choice for cleaning delicate surfaces. You would be surprised how much dirt and
algae this method will remove. It's cheap, and you can spray as much as you like
over yourself without harm. A five dollar packet of Lux soap flakes will make
fifteen gallons of creamy soap solution when it is mixed with hot water and
allowed to cool before use.

My next choice is neat oxalic acid used to spot clean tannin and rust stains.
Sprinkle some dry crystals on a damp sponge, dab gently only on the stained area
and rinse with clean water. You will need gloves for this- I use kitchen gloves
from the supermarket. Keep a bucket of water handy to rinse off any splashes.

If the algae recurs, it will need treatment with a biocide. I use 'Alginox' a
swimming pool algaecide ($15 for half a gallon, active ingredient 0.15%
Benzalkonium Chloride). Lightly wet the stone and apply undiluted, then scrub
with a soft broom until it froths evenly over the surface. Don't rinse - let it
dry on the surface. It will stain light brown for a few days, then lighten as
the algae dies and falls away. It has a strong detergent base, so don't get it
on yourself.

Regarding Portland cement stains, I have often used either neat or diluted
hydrochloric acid as a chemical solvent, or wire brush and water to mechanically
abrade the surface. The judicious use of up to full strength acid is
recommended, although you must take care not to let the acid dry on the surface.
It will stain as it oxidises, and the acid stains are very difficult to remove.
Rinse the stone surface with clean water before the acid dries completely.

I use a small bucket and hand brush to apply the acid in very small quantities
on the worst stains. I like to wear kitchen gloves and keep a bucket of fresh
water handy to rinse splashes off my skin. I find this method more effective
than having the acid held against my skin by protective clothing. Particular
care must be taken with the gloves. If acid drips inside them, it is worse than
not wearing gloves and putting your hands in acid. Rinse inside the gloves
regularly by dipping your hands in water while wearing them.

A wire brush is good for spot cleaning small areas that acid won't clean. Use it
on wet stone, and be careful to rinse all loose bristles and loosened dirt with
clean water. Don't use a wire brush together with acid, the bristles will fall
out very quickly.

Regarding the use of a pressure washer, it is easy to say just don't use it for
fear of damaging the stone surface, but this machine can be a very handy tool in
the hands of the right operator. A fan jet is essential, as well as training in
correct use. There are horror pictures somewhere on the Internet showing what a
jet can do to an eyeball. It's important to maintain the correct distance from
the stone at all times. This is difficult, because it's natural for human arms
to swing an arc against a flat building face, especially when you are suspended
from a swinging stage or cherrypicker.

One time, I worked with a group of stonemasons cleaning a major city building.
We had nearly reached street level after starting cleaning at the top - the boss
appeared one day and sacked an apprentice holding the water jet. He regretted
letting the apprentice finish his day on the scaffold because he etched "FCUK"
(yes he was dyslexic) in ten-foot high letters that were impossible to hide.

Simon