Floor plans

Here’s how to avoid costly failures when epoxy-coating concrete

by Paul Oman

If there is a trick to achieving a successful coating job it is to understand why coatings fail and how proper surface preparation can prevent those failures.

It’s hardly a small issue. Premature or immediate coating failure can cost tens of thousands of dollars in labor and material costs, as well as downtime.

Here is what you need to know.

Why coatings don’t stick
There are many reasons coatings fail to stick to their applied surfaces. Unless otherwise stated, let’s assume we are dealing with a concrete floor or loading dock.

Dampness: Moisture is a good starting place for a coating failing to adhere. Moisture doesn’t just mean water droplets; it could include high humidity. Some coatings shouldn’t be applied when humidity levels are high. More obvious sources of moisture include rainstorms during an outside application, standing water puddles on a concrete slab or, even more likely, a damp or saturated surface.

Surface dry doesn’t necessarily mean dry. There often is a high moisture content hidden just below the surface. The standard test is to tape a 4 foot by 4 foot plastic sheet to the concrete and see if visible moisture collects under the plastic. You can apply some modern epoxies to wet or damp surfaces, but generally a moisture-rich surface means no possibility of coating.

Moisture flow: Migrating moisture, as opposed to simple standing water, creates a more difficult problem. The common sign of this kind of failure is water-filled blisters. Just a tiny amount of flow pressure under a still-curing coating can ruin the bonding process taking place. There is no good answer here, but rapid-drying/curing coatings have a better chance of working. They can set up and bond before water and water pressure builds to unacceptable levels under the fresh coating.

Greases, oils, etc.: Few, if any, coatings stick to greasy, oily, waxy surfaces. This includes many kinds of plastic surfaces. Oily surfaces are tricky. Just feeling the surface often isn’t good enough. Even on what seems like a non-greasy surface, many coatings bead up, leaving behind hollow, coatingless circles or voids. Hosing down, jet-blasting or grit-blasting doesn’t guarantee grease removal. Indeed, it will probably stay behind. Greasy, oily surfaces require a degreasing chemical to remove the film. 

Experts suggest washing down the surfaces using a degreaser and a stiff brush. It’s wise to make this a standing surface preparation step and wiser still to do at least two degreasing washdowns. However, even simple degreasing or more advanced hot steam and chemical systems may not work completely. Surfaces contaminated with animal fats seem especially difficult to degrease and successfully coat without physically removing or replacing an inch of surface concrete.

Dust/slime/loose rust: It’s common to see floor coating samples collected from peeling floors that are dirtier on the underside than on the top surface. The coatings stuck, but to the dust and dirt on the floor instead of to the floor itself. Hosing down the surface to remove loose materials works somewhat, but that also leaves much behind. The same goes for sweeping. The best approach is probably hosing down with as high of a flow of water as possible, followed by compressed air blowing to both dry the surface and remove any remaining wet/sticky dust. A quick, last-minute broom sweep wouldn’t hurt. Simply sweeping is at the other end of the spectrum. 

The applicator must decide how much time and effort to put into surface dust removal. Contaminants, including salts, can stick to your floor well enough to withstand a hose or brush, yet mysteriously pop off after painting.

Salts: Salts and/or minerals (deposited out on the surface from the curing of fresh concrete or from the evaporation of seawater on concrete or steel) can quickly ruin a coating. For starters, the salts work like dust and other contaminants, getting between the coating and the surface. These cause even epoxy floor paints to peel off easily, despite what appears to be a clean, nicely profiled surface.

Without moisture, salts tend to form crystals, which can interfere with bonding. Experts claim these salts actually perform in a chemical sense like a grease, damaging or destroying bonding. However, it gets worse. Salts attract water both from the concrete and through the coating. The result can be a water-filled blister that spreads and grows mechanically, destroying the coating-to-surface bond.

Why coatings fail
Simply not sticking to the surface is one of the more obvious coating failures and has been discussed in this article. Other reasons include:

UV: Ultraviolet radiation from the sun breaks down most coatings. Epoxies tend to yellow and chalk; other coatings, such as plastics, get brittle and crack. After too much UV, coatings fail.

Porosity: Coatings aren’t as hard nor as solid as you might think. Some coatings are quite porous, permitting moisture and chemicals to invade the coating and attack it and its bonding interface. Some epoxies are more porous than others. One common fix used by makers of porous brands is to add “glass flakes” (usually mica) to slow the migration of water or chemicals through the coating.

Pinholes: A single layer of any coating runs the risk of including tiny pinhole areas of non-coatings. These leaks permit chemicals to attack the unprotected surface as well as the bonding interface. In harsh environments, apply two coats of paint/epoxy. Each coat is a contrasting color to aid in spotting thin or missing spots.

Brittleness: Simply put, some coatings are more brittle than others, and overly brittle coatings can crack or chip with impact or expansion and contraction.

Abrasion: Different coatings handle abrasion and wear better than others. Some of the more wear-resistant epoxies now available contain Kevlar microfibers. These fibers, in addition to improving wear resistance, also work like rebar does in concrete, localizing any damage to the coating.

Chemicals: Chemical resistance varies between coatings. Strong acids, for example, quickly destroy many coatings.

Movement: Paint over an expansion joint and you can count on a coating failure. The slabs will move and the coating over the joint will crack and probably disbond chunks of coating in the process.

Salts: These were covered in the previous section. Besides possibly reacting like a surface contaminant, as well as acting like a grease, they also attract moisture through both the coating and concrete, forming water-filled blisters.

Surface preparation
How much surface preparation is necessary is a tough call. Someone has to take the responsibility, balancing effort and success against time and money.

An initial step is to insure the concrete’s strength and integrity. Crumbling concrete is a poor or impossible surface to bond with. There are several repair or replace options, including sealing the surface with a thin, penetrating epoxy or some other compatible product. Once that is done, real surface preparation can begin.

The process of surface preparation involves at least five steps:
1) getting rid of greases, oils, etc.;
2) providing a surface profile or texture;
3) removing dust and contaminants;
4) removing salts;
5) removing moisture.

Thanks to exposure to the elements, outside concrete slabs are often more ready to accept a coating than interior concrete surfaces. New concrete usually has a thin veneer or crust of very weak concrete and salts that absolutely must be removed prior to coating.

In a perfect world, prepare surfaces as follows:

1) Two washdowns with a degreasing chemical.

2) The use of a Blastrac-type machine that performs concrete surface grinding to remove weak surface layers and give the concrete floor an enhanced profile for bonding. Special grinder machines also work, as does a muriatic acid treatment known as acidizing.

Acidizing is an easy and inexpensive option. Muriatic acid is sold to clean concrete and bricks and is available in concentrations of 20 to 40 percent. Dilute it to 10 to 20 percent (add the acid to water; don’t add water to the acid). If the concrete/acid solution fizzes and the acid releases strong fumes, dilute the acid some more. New concrete will fizz more than old concrete. In any event, sweep the floor twice with a thin layer of muriatic acid. Rinse off the acid-washed surface well. Check local disposal rules regarding this nasty waste liquid. 

3) Ideally, the next step is a high-pressure waterjet cleaning, hopefully at pressures of around 3,500 psi (at pressures over 5,000 psi, waterjetting will actually start to remove concrete, and could be an alternative to Step 2). After waterjetting, wet-vacuum the surface dry, sucking up the remaining water and dirt. Sweep well before coating.

This step often gets reduced to simple sweeping and air drying. In most cases, that’s probably OK, but every shortcut raises the chances of problems.

4) The concern about all the bad things various salts can do to about-to-be-coated surfaces is growing as their effects are better understood. On steel surfaces, these salts form ions and corrosion cells that are easy to spot because of the rust. Concrete may have a bigger problem than steel since it is porous and contains minerals to start with.

Several newly developed chemicals and/or processes deal with these salts. Seriously consider using or testing these chemicals. Some applicators now include desalting treatment as a standard step in surface preparation. Often, the recommended method of application is with a waterjet unit, however you can apply some with a brush or roller.

Pre-existing coatings
Most surface-tolerant, modern, solvent-free coatings can be applied over well-adhered traces of a pre-existing coating. “If it ain’t broken, don’t fix it,” may be a completely valid strategy. On the other hand, complete stripping and total surface preparation is the absolute proper approach to take.

Vertical surfaces
Vertical concrete surfaces are less likely to have a grease film or thick layers of contaminants than floor surfaces. They often also have a pre-existing rougher surface, negating the need for blasting a profile into the concrete. Salts and loose/crumbling surfaces, pre-existing coatings or moisture are probably the major problems with coating these surfaces.

Conclusion
Coatings fail for many reasons, but proper surface preparation can often prevent failure. That said, proper surface preparation is often given short shrift to save costs, time or materials.

Because each coating situation is unique, sometimes cutting a few corners causes no problem. It is a gamble played out every day between end-users and the coatings they use. It is one thing to gamble and lose and quite another to fail out of ignorance. s

Paul Oman is the co-owner of Progressive Epoxy Polymers Inc., a supplier of paints, coatings and related products. For more information, call 603-435-7199 or visit www.epoxyproducts.com. 

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