There are certain inherent problems when painting previously painted concrete decks or walkways.
Typically most ground floor concrete decks did not have a vapor barrier placed on the ground under the concrete deck prior to the pouring of the concrete slab. Moisture can then migrate through the concrete. The hydrostatic pressure and vapor gets trapped at the interface where the paint film meets the concrete. The tough hard finish of most deck coatings will not allow this moisture (vapor) to pass through the paint film, and thus the pressure will force the paint away from the surface, causing peeling. The more coats of paint the greater the propensity for this to occur.
Previously painted concrete decks above the ground also have inherent problems when being repainted. The multiple coats of paint that have been applied over the years can cause a phenomenon known as intercoat adhesion failure. The more paint applied to the surface, the less the paint can breathe. When a subsequent coat of fresh paint is applied to a previously painted deck it may cause a bubble or blister on the painted surface. The cause of the blister is because water based or latex paints dry by evaporation. As the floor coating dries and begins the evaporation process the wet film is shrinking (loss of water) and a “shear” is being placed on the previous coats of paints that were applied. To further compound this problem the wet paint contains various chemicals that may soften or re-wet the underlying paint. In both cases when a deck has numerous coats of paint you can almost bet the next coat of paint will cause the decks to blister, bubble, or peel. The newly applied deck paint is only a good as the adhesion of all the previous coats of paint.
” A chain is only as strong as the weakest link”
Therefore, when either of the above phenomenons occurs that is an indication that the decks will need to be stripped down to original substrate. There are various types of processes that can be used to strip off the old coatings from a surface. Wet sandblasting, shot blasting, scarifying, or high water pressure are typically the best and fastest methods. Scarifying or grinding may be the least invasive process to properly prepare decks for long-term adhesion when coating previously painted decks.
If peeling, blistering, or bubbling occurs on previously painted concrete decks it is usually not the fault of the newly applied coating system or contractors applications procedures. It is a typical phenomenon associated when painting previously coated concrete decks.
If you should need any further information or have any additional questions please contact us.