Removing Stains on Masonry Surfaces Q&A

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Dear NH,

There are green chlorophyll stains all throughout my mortar joints on the front brick stairs. I have made several attempts to clean them with muriatic acid and wire brush. It seems to work for a while but the stains come back. Can you offer any more permanent solution?

CM from North Easton, MA

Dear CM,

You are stripping off (actually dissolving away) a little of the mortar with each muriatic acid treatment, inadvertently providing a good medium for regrowth of the moss (or whatever plant life it is!). The muriatic acid leaves the mortar slightly more porous (and slightly weaker, too).

Here is a suggestion; use a phosphoric acid masonry cleaner followed by a neutralizing rinse of household ammonia and water... 1 cup of ammonia to 1 gallon of cool water. Let the area dry thoroughly and apply an exterior grade clear masonry sealer. There are a number of products available at your hardware or home store. This should seal the mortar sufficiently to retard further growth. However, I cannot guarantee that this problem will not eventually recur, especially if the mortar is in a very damp location. Plan on cleaning and resealing the mortar annually. Don't use the muriatic acid, though... use the lower strength, less damaging phosphoric acid cleaner.

There is a new spray-on product that increases resistance to mildew and algae on outside surfaces called Moss Out (available online and at many hardware and home stores). It utilizes a zinc mixture that both kills moss and mildew but leaves a protective layer that can last up to a year, depending on the climate. Way cool!

Dear NH,

A bottle of muriatic acid has leaked onto our concrete floor in our basement. I did not realize it happened until I saw the area of concrete discolored, rough and porous. Is there anything I can do to restore the appearance and texture of the concrete without having to pour new concrete in the problem area. I have asked experts and they have told me to use a 10% ammonia solution to water ratio and a straight ammonia pour onto the concrete, but both ways didn't work. I would greatly appreciate your input as to what I can try as to no one knows the answer.

NC from Chicago, Illinois


It was correct to apply the ammonia solution, since it neutralizes the muriatic acid to prevent further damage. The ammonia was not a repair, though by the tone of your letter you seem to have thought it would restore the concrete. It won't.

The repair material you will need to use will depend on the extent of the degradation of the concrete. If the surface is pitted or flaking, all loose material should be wire-brushed, chiseled or scraped out. Then you can smooth the surface with a cement topping mix. Topping mixes are designed to be used in very thin applications and can be spread to a feather edge since they contain very fine aggregate particles.

If the deterioration is severe and there is an actual hole of more than 1/2" in depth, you should probably use a concrete patching compound, which is similar to the topping mix but has more strength in thick applications. Don't get a fast setting mix... it is unnecessary for your application and frankly more difficult for a novice to work with.

There are some very sophisticated epoxy patching and resurfacing compounds that will also do the job. They are not cement-based but do stick like crazy in difficult situations.

You will not be able to restore the concrete to the "exact" appearance, since repairs rarely look like the original work due to color variations, regardless of the skill of the mason. The best you can hope for is a patch that is smooth and blends somewhat with the existing work. Of course, if appearance is critical, you could have the entire floor resurfaced. If the patch is professionally done, it will disappear under the new surface coat. As an alternative to resurfacing the concrete to gain a more uniform appearance, you might want to consider using a quality concrete floor paint.

Dear NH,

I need suggestions on the topic of my natural stone fireplace. I want to know how can I effectively lighten the stones, possibly without painting them. The fireplace is very large, from floor to ceiling, and is the focal point of the living room. It is not dirt or soot, it is just that the stones are quite dark, and my living room is on the southwest side of the house, so during the daytime the room is dark. I have painted the walls, bought new furniture, curtains, etc., and I feel that lightening the stones would really bring the room to life. I was going to whitewash them, but my husband had a fit. He said that they are natural stones and that painting would ruin them. He may be right. But my living room is still so dark. Let me stress that the stones are not sooty or dirty, just naturally DARK. Would appreciate some feedback.

PM from Charlotte, NC


Your husband may be right depending on the type of stone. However, whitewashing is not intended to be a temporary fix... it is supposed to be a permanent change to the stones. Done correctly, it offers highlights and contrasts... not a head-snapping dose of white paint!

It may be possible to lighten the stones somewhat by treating them with a masonry cleaner containing either oxalic or phosphoric acid. These relatively mild acids will clean any unnatural discoloration from the stones, but I really can't guarantee you what the result will be. It could be a minor improvement or no improvement at all.

Any clear coatings you might want to apply will have either a neutral effect or cause a slight darkening. I am unaware of any coating that will brighten stone. Add a slight gloss, yes, but brighten... no. There are slate and stone sealers available at most paint, hardware and home stores.

So if you really need the wall to be brightened up more than a touch, a professional whitewashing might be the way to go. Since the results are permanent (unless you want to get involved in a very messy and/or dusty paint removal job), I would suggest having a few professional painters with experience (and references) in this type of work take a look and offer suggestions. Perhaps, through more knowledge, a middle ground can be found between your desire for more brightness and your husband's understandable concerns.