Thursday, August 19, 2010

How to Prep Concrete for an Acid Stain

By Ted Uram

It's really hard to find anyone who can't appreciate the wicked coolness of an acid-stained floor. And while the colors are not as plentiful as what can be achieved with other, non-reactive staining alternatives, nothing beats the subtle complexity of the acid-staining concept – acid stains literally burn in a lasting coat of beauty. And they achieve this in a very simple way.

With all the fluxuating trends and shifts in floor coating technology, acid stain floors continue to remain popular. Why? Simply put, no other floor technology achieves results quite like acid staining – that mottled look, that inconsistent yet strangely appealing speckle of color. Try and faux this and you will be hard pressed, try a non-reactive alternative and you may be let down.

What's Going On?

To understand the prep for an acid stain you must first understand how all of this is happening. Here's the basic concept:

Acid stains are made from blending small amounts of reactive metallic salts with acidic agents, such as muriatic or hydrochloric acids. The metallic salts provide the color while the acidic agent burns it in. But what does all this really mean? What is really happening?

The uppermost layer of concrete in good condition contains a concentrated crust of cement paste that is just not present in rest of the slab. This occurs during the concrete finishing process when excess water is bled to the top, and the top portion of the slab floated to make it nice and smooth. It is this thin layer of material that is reacting with the acid stain – not the rest of the slab.

I'll say it again – the uppermost crust of the concrete slab is where acid stains react.

Okay. Then the rest should be common sense. Right?

Read on...

What Went Wrong?

Talk to any coatings guy (or gal) and you will hear the same thing: surface preparation is key to any successful coating. How true. But sometimes, in an effort to simply do the right thing and get that surface prepped really really well, some folks can overdo it.

Take this into consideration:

“I don't understand! I did everything right. The floor was in pretty good condition. Had a few spots where something got on it, but I cleaned it all up really well, scrubbed it good, and then acid etched it according to ASTM standards. The floor had a really good profile. I applied the stain and it looked great. But when it came to neutralizing the floor, there was hardly any color there! What did I do wrong!?!”

See what I mean? Based on what we've discussed, can you spot the error?

It's not the applicator's fault. For just about any other coating this would have been a superior concrete prep job. But what the applicator failed to take into consideration was the application itself. Acid stains react with that uppermost layer of concrete. When the floor was acid etched, most of the reactive layer was already burned away.

Another example:

“I just can't understand what went wrong. We're always really good about prepping the surface. I've been doing coatings for years and we always make sure and get a real good surface profile. We clean the floor, making sure to degrease and scrub out any oil spots, scrape everything up real good. We always take an orbital floor grinder and grind down to make everything nice and even. But when we applied the acid we got no reaction at all? What went wrong?”

Starting to get the picture? Again, not an intentional error, but an error nonetheless. Get too aggressive with the concrete and you literally strip away the very stuff you need for a good acid stain reaction to occur.

How about this one:

“This just doesn't make sense. The concrete is practically brand new. I mean, it's had a few months to cure, so that shouldn't have been the problem. The concrete looked great. We knew enough not to acid etch the floor since we were using an acid stain. So we took a floor machine and scrubbed the whole surface real good with a mild cleanser. We rinsed it real good, several times actually, just to make sure there was no soap film or anything left behind that might react with the acid stain. The color went down beautifully. But when we neutralized, all of the color literally washed off. It was like we never even stained at all!”

This one might be a little harder to spot. While it's true that fresher concrete is usually a better candidate for an acid stain, there can often be a hidden danger here. A surface sealer. Surface sealers are sometimes sprayed down by the concrete crew to help keep moisture in the slab so that it can achieve a good hard cure. Surface sealers are often used in drier, hotter areas, for reasons that should be obvious. But surface sealers can also deflect coatings as well as stains.

The solution in this case? Well... get ready to go slightly against the grain. But just slightly.

Before we talk about the solution for this one let me just state that a very simple test can help you tell if a surface sealer is present. Just sprinkle some water on the surface. If it spots up and sinks in you're usually okay. If it just beads you've probably got a surface sealer.

Okay, that solution: In this case you actually MAY need to acid etch the surface in order to break up the sealer and remove it. If you have to do this use only a VERY LIGHT CONCENTRATION. You don't want to burn away that reactive material. But before you do that, try using any common surface stripper. Scrub it in with a floor machine, rinse and allow to dry. You should be good.

So What do you Recommend?

Just clean the floor real well. Use common sense and remove any adhesives, degrease and scrub any oil spots, etc. If the surface looks uneven or pitted, or if there are marks on the slab from the builder, or spots where carpet adhesives, tape marks or glues have been, then you may want to consider just laying a good overlay and staining that. Overlay material usually stains very very well. Otherwise, just let the concrete and the acid stain work together.

Some Insurance

Here's a surefire way to tell exactly how your floor is going to react. Actually stain it. But only in a 1' X 1' square. After you've done your cleaning, degreasing and rinsing, use some poly tape to tape off a small square (a closet is a good spot) and actually apply some stain. Go on to your next job and leave it overnight. When you come back the next day (don't go more than a day) wipe it with a rag dipped in ammonia water (wear gloves). You'll know right away what kind of color burn you are going to get.

Was this information helpful? I certainly hope so. You can learn all about acid staining floors and more in The Book of Decorative Concrete Coatings. We cover every aspect of acid staining, as well as all kinds of other popular decorative concrete coatings.

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