Surface Failures---"Hollow Spots"

Among the many causes of surface failure and reasons pool interior finishes require resurfacing is de-lamination, also called “hollow spots” or “pop-offs”. When surfaces do fail, there is a tendency for all parties to finger-point and deflect blame. In this case, the failure of the finish to stick to the substrate is, without a doubt, the applicators fault. There are various conditions that can cause hollow spots and all are within control of the contractors and subcontractors. The reason finishes don’t stick is relatively simple to understand once you realize all cement surfaces are porous to some degree. They absorb moisture at different rates depending on the density of the material. The “softer” the material, such as old pool plaster, the more water it sucks up. When a newly applied wet cement finish is applied over an old cement surface, the old surface absorbs the moist cement content of this material at the place of contact. This cement forms the bond between the two materials. If the old surface draws out too much of the cement at this contact area, the bond is compromised. There will not be enough cement in place to form a tight bond. In areas of de-lamination, usually varying between six inches and a couple of feet wide, the finish will pull away from the old surface about a quarter to a half-inch. Sometimes hollow spots are not detected when the pool is full. If the finish is applied thick and dense, water cannot get behind the spot and lift it further away. Sometimes, simply the weight and pressure of the water will keep a hollow spot from lifting. When hollow spots lift away further, they tend to crack allowing water to fill the void. This water gets saturated with calcium and calcium salts will form along the crack. That’s what I look for when I bid on a resurfacing job. Although it is more common to find hollow spots on previously resurfaced pools, I come across lots of new pools with that problem. (The record number of hollow spots is 40 in a 450 sq.ft.) Did you check out those pictures in the disaster files? Among some of the other reasons for hollow spots in new pools are dirty surfaces and “hot spots” on the concrete shell. These are areas that have a higher sand content at the surface. Gunite pool shells are very porous and spotty. I anticipate lots of work in the future due to bond failures in new gunite pools in this area. 

So how do I prevent hollow spots from occurring? First of all, all my pools get a thorough cleaning. This will allow a good surface for an application of an acrylic-based bond-coating surface. This bonding surface has sealing properties that will prevent too much cement to be drawn out of the newly applied finish, thus providing excellent adhesion. Before this bond-coating was used, pools were either acid-washed or sandblasted. Just because a surface is rough doesn’t mean something will stick to it! Bond-coating a sandblasted pool may seem redundant, but I learned the hard way and it cost!  

Of course, right before a new finish is applied, everybody knows you’re supposed to hose down the pool walls and floor to cool them down and moisturize. This will allow adequate bonding, right? Let me tell you a secret. The subcontractors that apply pool finishes, whether they are new pools or re-dos, get paid by the job, not hourly. So, if they can get the pool done in less time, do you think they will try? Of course! The first thing the finishers will do is NOT wet down the pool surface. This easily knocks an hour or so off the drying time. Aren’t these guys compromising the bonding? Yup.  What do the contractors do about it? They point the finger. What do I do about it? I’m in every pool I do, and I’m the one who guarantees it for five years. So I make quite sure everything is proper and as good as it can get. It is common knowledge that one of the popular subcontracting firms in my area never wets the pool down before they apply the finish. I’m looking at a lot of work in the future thanks to these guys.

Pro-Cite is located in southern Brevard County Florida which includes the communities of Melbourne, Palm Bay, Viera, Suntree, Satellite Beach, Indian Harbour Beach, Indialantic, Melbourne Beach, Eau Gallie, West Melbourne, Malabar, and most of the Space Coast.

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Pro-Cite, Inc.
P.O. Box 372457
Satellite Beach, FL 32937