Paint outlets often have displays showing highly flexible paint films used to waterproof flashings and parapets. While these paint films appear to be very impressive to customers, there are a few facts worth taking into account before making a purchase. High flexibility does not equal quality or ability to waterproof a structure. High levels of flexibility in a waterbased coating are achieved either by use of a 'soft' binder, or by partial replacement of the total binder loading with a rubber latex. Rubber latex containing acrylics must always be over-coated due to poor u.v. resistance properties. As u.v. light breaks down the rubber latex, the coating becomes porous and ceases to function as a waterproofing barrier. The degree of flexibility required by a water-proofing coating needs only to be sufficient to absorb the difference in expansion and contraction rates between the different substrates it is applied to. In the case of flashing, this would be the metal flasing and the plastered parapet or roofing material. Soft pure acrylic binders can absorb this type of movement, and do not require over-coating, as they are u.v. resistant. Check with your supplier as to the binder type used to produce the coating - the correct choice will save time and money in the long run.
