Ask the Carpenter: How to clean stains off your bluestone patio

Ask the Expert
Plums bombard and stain a reader’s bluestone patio. Sealant would make cleanup easier.
Plums bombard and stain a reader’s bluestone patio. Sealant would make cleanup easier. Handout

Q. Last year we had a landscaper do our backyard and install a bluestone patio. It’s now being attacked by plums that fall, splatter, stick, and stain. I knew we would have to deal with plums every year but never realized it was going to be this many — perhaps because California has suffered droughts for the past few years.

We are trying to minimize the problem, such as picking the fruit as much as possible before it falls and discussing pruning with an arborist.

It’s very hard to clean up the fruit, because they get so stuck to the stone. It’s even difficult with a hard scraper, because the stone is not perfectly flat. I also tried using Simple Green and a stiff deck brush. This helps a little, but the stone remains stained. Are these penetrating stains that won’t come out? And even if they can come out, do I need to clean the stones right away? I am trying to figure out whether I need to clean them weekly when the fruit is in season.

I am seriously thinking of getting an electric pressure washer to make the job easier. What are your thoughts? Thank you so much for helping.

YG, San Francisco

A. The stains are difficult to get out, but I think using a pressure washer and a bluestone cleaner will do the trick.

Pressure washing

Before you buy a pressure washer, rent one and use a store-bought detergent to see whether you can get the patio clean.

Sealing your patio

After getting the patio clean, seal it. Most sealers on the market will work on bluestone. Consult the package to make sure you get the right kind, and then you can apply it using a standard paintbrush. Use light strokes when painting on the sealer, and then wait a day for it to dry. Once the sealer is dry, apply a second coat.

Using acid wash on difficult stains

As a last resort, I’ve used muriatic acid on my bluestone patio. Do-it-yourselfers should avoid using muriatic acid whenever possible. Use it only after exhausting other cleaners like trisodium phosphate detergent or less caustic concrete-stain removers from your hardware store.

Muriatic acid is not the first choice for masonry cleaning; it’s the last.

Muriatic acid is a highly reactive liquid acid and one of the most dangerous chemicals you can buy for home use. It is an industrial-strength solution of hydrogen chloride gas dissolved in water, also known as hydrochloric acid. With the exception of some plastics, muriatic acid can damage most anything it touches, including clothing, metal, and skin. It emits a suffocating odor that can quickly burn the lining of the nose, throat, and even the lungs. If you consider using muriatic acid, please heed all safety recommendations both here and on the product label. Muriatic acid must be used with extreme caution! Contact with the eyes, for example, can cause permanent blindness. Contact with the skin can cause severe burns.

Using muriatic acid

Dress appropriately: Wear safety glasses, acid-resistant gloves, long sleeves, and pants, and use a respirator approved by the National Institute for Occupational Safety and Health that is equipped with the appropriate acid-grade filter.

Have a neutralizing agent and a reliable, steady source of water available. Baking soda or garden lime can quickly neutralize the acid if spilled. Water should be freely available in case you accidentally get acid on your skin. Since muriatic acid can damage or kill foliage, cover or wet all plants in the area with water before applying the acid. Cover them with plastic. Do not use a sprayer on a windy day.

Work in an area with adequate ventilation. Use a fan to bring fresh air to the work area if necessary. Muriatic acid is nonflammable, but the vapors are highly corrosive and irritating. Using muriatic acid indoors is not recommended. The corrosive vapors can begin chemical reactions in metals, leading to permanent damage.

If you spill muriatic acid

Spreading a generous quantity of lime (the powdered or crushed type used for lawn or gardens) or baking soda and adding water will cause a distinctive fizz as the lime reacts with the acid. Garden lime is less expensive than baking soda and is sold in larger bags.

Muriatic acid should NEVER be poured down a storm drain or a sink or flushed down a toilet. Doing so can cause extreme damage to pipes, dissolve solder, and disrupt the biological balance of your septic system. Throwing even a closed container of muriatic acid in the garbage can be dangerous for trash handlers, damage their trucks, and cause unexpected chemical reactions in landfills.


■ Muriatic acid (1 quart for small areas)

■ A respirator

■ Safety glasses

■ Acid-resistant rubber gloves

■ A garden hose with a high-pressure nozzle sprayer

■ Water

■ Lime or baking soda

■ A long-handled masonry scrub brush

■ Plastic sheeting

■ A paintbrush

■ Nonmetal sprayers (if you choose not to paint it on)

Preparing the mixture

Always pour acid into water, never water into acid. Mixing the two causes a reaction that gives off heat. This reaction is much more sudden and violent when water is poured into acid. Mix the acid and water in a plastic container. The mixture should be diluted to at least 1 part acid to 10 parts water.

The diluted acid can be applied with a brush or sprayer.

Steps to clean rust off masonry

1. Wet the stain and the surrounding area.

2. Mix the acid with water. One part acid to 10 parts water (by volume) is typical, but dilutions as light as 1 part acid to 16 parts water also work well. Note: A 1-to-16 ratio is 1 cup acid to 1 gallon of water.

3. Read the product label and follow the recommendations.

4. Brush or spray the acid onto the affected area. Do not use a metal sprayer. A plastic sprayer will work for a while but will eventually be destroyed by the acid. Have a few extras nearby and dispose of used sprayers.

5. Let the acid sit for no more than a few minutes, less if you can see the rust lifting.

6. Scrub off any residue with a stiff brush while rinsing thoroughly with water. There are long-handled masonry brushes ideal for this job.

7. Neutralize the acid with lime or baking soda and rinse.

8. Rinse again.

For stubborn stains:

Apply a small amount of undiluted muriatic acid directly to the stain and repeat steps 5 through 8. Always rinse thoroughly.

Be aware that this process may leave your concrete or bluestone looking cleaner than the surrounding area. To minimize this, work only the stain and rinse quickly. The longer the acid mixture is left on, the more it will clean — and possibly etch the surface.

Safe disposal

1. You can use lime to neutralize leftover muriatic acid. Use a large bucket to minimize the chances of dangerous spattering; I prefer a 5-gallon one. Put three or four cups of lime in the bottom and a gallon of water. Give it a stir.

2. Put on your respirator. Slowly add the acid to the bucket, keeping your face away while you pour.

3. Stir, adding more acid and lime until all of the chemical fizzing has stopped.

4. The fully neutralized acid can now be safely disposed of down a sink or storm drain without fear of damage to your septic system or the environment.

Rob Robillard is a general contractor, carpenter, editor of, and principal of a carpentry and renovation business. Send your questions to or tweet them to @robertrobillard. Subscribe to the Globe’s free real estate newsletter at