Tackling the Hidden Sins of Aged Floors

MONDAY, JULY 7, 2014

Floor projects can be tricky, even when the slab is new and the design specs are mindful of best practices for coating.

And the challenges multiply when an existing building is renovated and/or re-purposed, and its bare, aged floor is slated for a handsome new coating.

Problems can start at the bottom. If the surface was never meant to be coated, the slab may be placed directly over gravel or sand with no vapor barrier — and any coating designed to protect the surface will fail if a moisture problem already lurks below.

Another concern is that the slab may have been troweled too smoothly and will require careful surface preparation to achieve the necessary profile for proper coating adhesion.

And, sometimes, the floor may have been sealed with non-visible surface treatments or contaminated with years of oil and grease. Any of these can cause problems with adhesion, curing, or application, and will also prevent acid etch solutions from properly roughening the surface.

From Welding Shop to Showroom

Such was the case when the inspector was called to a building where a 20-year-old welding shop was gutted and converted into a car dealership. The bare concrete floors received a decorative finish.

Two different floors were coated by two different contractors using the same epoxy finish. Years later, one floor remained in excellent condition. The other failed so fast that the tires of new cars started picking up sections of the peeled floor coating when they were driven into the showroom.

What happened? That's what the inspector wanted to know.

Surface Scrutiny

The first task was to check for pre-existing treatments on the floor, such as moisture-retarding sealers applied while the slab cured, or clear sealers applied to reduce dust.

(You can ask for records from the building owner, but frequently, no history is available. Then a surface inspection is required.)

If the surface is visibly shiny, that’s a sure clue. But since heavy traffic may have worn away the treatment in some places, be sure to look in low traffic areas (corners or under equipment) as well.

Also, many treatments are invisible to the eye, so a surface test may be required.


One substrate, one coating, two locations: One project worked; one failed fast.

The most popular is the simple water drop test. If the water drop beads on the surface and is not absorbed by the concrete, there is a good chance that the concrete has been treated.

A sure test is to apply a muriatic acid solution to the surface; if there’s no reaction (bubbling, spitting, foaming), the surface has a treatment that must first be removed, typically by mechanical surface preparation.

In the case of the welding shop, the floor surface did not have a sealer, but it was stained extensively from cutting oils and carbon residue.

Such contamination can wreak havoc on a coating job by:

  • Preventing the acid etch solution from properly roughening the surface; or
  • Interfering with adhesion of the coating later.

Floor 1: Secrets to Success

The first floor was washed repeatedly with an emulsion cleaner to break down the oil; after each washing, a sample of muriatic acid wash was applied to see if a reaction ensued.

If the solution didn’t foam, the surface wasn’t yet clean enough, and another round of scrubbing and hosing down with cleaner was required (done in accordance with local environmental regulations).


Testing thoroughly for non-visible contaminants or surface treatments during surface preparation will help ensure a quality coating job.

When, after repeated cleanings, the surface was deemed clean enough, the contractor did an acid etch prep with a muriatic acid solution until the surface was dull and gritty to the touch (like 100 grit sandpaper). The surface was then thoroughly rinsed, dried, and tested to verify that the pH was within an acceptable range for the coating system.

The decorative epoxy floor coating was then applied, using a base coat followed by a clear with decorative chips broadcast into the wet surface, followed by two more clear coats.

This floor came out beautifully.

Floor 2: Formula for Failure

After the first project, a second floor in similar condition was coated by a different contractor without an inspector present. This floor began peeling almost immediately.

Failure analysis determined that the only surface preparation had been solvent washing in an attempt to lift the oil. That was the first red flag. Hydrocarbon solvents such as mineral spirits, MEK and xylene should not be used to remove grease and oil because they may spread and transfer these contaminants deeper into the concrete.

But in addition, no acid etch solution was ever applied to roughen the surface.

Acid Etch

Muriatic acid application can reveal invisible treatments on concrete floors and should be applied to roughen the surface for coating.

In our experience, inadequate surface prep is frequently the culprit when a floor coating starts to peel shortly after application.

Sometimes, surface preparation gets compromised by the owner's relentless pressure on the contractor to meet the building opening deadline — pressure that is exacerbated when the construction schedule is prepared before the extent of the floor contamination is fully recognized.

Better Safe...

With a project like this, an owner is better off allowing time for proper surface preparation. The dealership incurred considerable delay and expense moving the cars offsite while the peeling coating was scraped and grinded off, the surface properly prepared, and a new coating applied.

In summary, bare, aged concrete floors can hide a multitude of flaws that, if not identified and rectified, can quickly lead to premature failure.

To remove any doubt about the cleanliness of the surface, start by preparing a 3 x 3 foot test patch. Apply one coat, let it cure, and then perform a crosscut adhesion tape test according to ASTM D3359. If the adhesion measures 5a, the rest of the job can proceed.

About the Author

This article was written by PQA Inspector Dave Lick and is reprinted with permission from the MPI (Master Painters Institute) newsletter. MPI content describes best practices for commercial, institutional, and light industrial painting.


Tagged categories: Acid etching; Coating failure; Concrete floor coatings; Failure analysis; Inspection; Maintenance + Renovation; Master Painters Institute (MPI); Surface preparation; Surface profile

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