Issue 30
August 31, 2005

Feature Article
The Outdoor Dilemma
by Gaye Goodman

The Outdoor Dilemma
by Gaye Goodman

We have been receiving a number of calls this summer about the pros and cons of acid-staining concrete surfaces which are outdoors.  We have written about this subject before but would like to revisit the topic for many of our new stainers.

Can I stain outdoors?

We found that the acid stain outside is not a problem, but the sealers can be. In the high desert of New Mexico, which has strong ultraviolet light radiation from the sun, both water-based acrylic sealers AND the more expensive solvent-based acrylics can start to blister and flake, just like your skin when it gets mildly sunburned. This can occur at altitudes of 5,000 feet (the level of Albuquerque) after a few months, and at altitudes of 7,000 feet (up in Santa Fe) just a few WEEKS after application. In fact, our xylene-based sealer began to blister sooner than the water-based and was harder to repair.

Is there any difference in the sealer that should be applied outside vs. inside?

The sealers are necessary to protect the stain, since rain can "reactivate" the stain and make it wash away, flowing downhill and streaking. I have had clients suggest that we eliminate sealer altogether, but I would not want to guarantee that the concrete stain would not fade or wash off after a while.

The sealers for acid stains are designed to "breathe" so that water can rise out of the new concrete slab for a few years without creating efflorescence over your stain color. However, that permeability is a two-way street. While acid can bleach your stain right through the sealer, spilled olive oil or fat from the barbeque can just as easily create a darker spot under the sealer, as it penetrates the stained slab and makes it appear permanently wet.

Most sealers will tell you on the label if they are meant for outdoor as well as indoor use.  If you are not sure, I would call the company. Most janitorial supply stores carry a good hybrid acrylic sealer which serves as topcoat and final finish and can be used indoors and out.

Do I apply wax to my outdoor stained concrete?

Most acrylic floor waxes can NOT be used outdoors, as they can’t survive wide swings in temperature.  We usually use the same sealer indoors and outdoors, then just wax indoors.  I use the term “wax” loosely.  The janitorial supply companies call these products “acrylic floor finish”.

Will the concrete be slippery when wet?

We only accept those outdoor stain jobs which are covered by roofing or shade, and if the concrete has a smooth finish, I warn the client in writing about the "slippery when wet" problem, and give detailed instructions for sealer maintenance, as well.

Stain manufacturers all warn that outdoor slabs should be broom-finished or otherwise troweled for slip-resistance. This helps, but the top ridges of the broom finish then carry all the foot traffic and can be scraped gray by abrasion, while the low places, where stain and sealer are not walked on, look even darker by comparison. I do however recommend this or a stamped slate pattern for areas with snow and ice. Staining over this will result in a nice mottled pattern. Some manufacturers also carry non-skid sealer additives, however I don’t find these to be good enough to be used outdoors. I have heard that they soon get “walked out” of the sealer, so you end up with a smooth, but pitted sealer.

Can I stain around a pool?

Staining near a swimming pool present the same problem of slip hazard, with the additional problem that the strong acid, used to balance the pH of pool water, can penetrate your sealer and bleach out the stain color in spots or rings. In fact, when we accidentally spill some stain color on a gray driveway during our work, we remove it later by acid-etching through it with a ten-to-one solution of water and muriatic acid. This is much less strong than the undiluted acid which is likely to run down the side of the pool-tender's bottle when he sets it down on your stained pool border

Will the stain be damaged by UV light at high altituded?

If you live in an area at a high altitude, like I do in Albuquerque, you will want to take UV light into consideration.  The strength of the UV light here can be a problem, and if your patio is in full sun you must be prepared to reclean and reseal it about once a year to keep it looking good.  I have found that all the clear sealers get “sunburned” at high altitudes, even though they claim to be “UV resistant”.

Do certain colors work better for outdoor use?

We have greatly reduced the number of outdoor jobs we do, and those rare clients usually opt for brown. I have found that greens tend to fade, but have not had that problem with blues. If the cement isn’t dry the water can leak up and cause black spots to appear in the green and blue stain. I did, however, stain the bowl of a large outdoor water fountain for a friend once. He wanted it blue. We sealed it with Cementone (water-based acrylic) and did not get one black spot! From this I concluded that water on TOP of blues and greens does not cause a problem--but that was just a small, unscientific sample.

How do I stain close to the house without damaging the exterior wall?

You might encounter problems with staining outdoors in masking the walls of the house which adjoin your stain area. Stucco can only be masked with colored duct tape, so masking becomes more expensive and laborious. Gray duct tape has too much tackiness, and regular masking tape does not give enough. You may also need to clear away dirt and mulch piled up against the outdoor slab, and stain all the edges, so as to give the impression that the stain color runs completely through the slab. This takes more effort, but gives the slab a nice solid look.

I do not want to completely discourage you from staining outdoors.  We touch on this subject a bit in the book, however, these are a few things to consider before you stain outside that you don’t have to worry about when staining indoors.  Staining outdoors is a quest we are continually looking into improving, and since it is of great interest to many stainers during the summer months, if we discover anything new we will share the findings. You will also find that this topic is brought up on the forum continuously, so check back periodically for any new information.

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Bridgeworks, Inc

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