Some people have asked us if elastomeric paint on their monolithic / texture cladded home is the best option. So, what is elastomeric paint? Thank you to Colin Gooch, Resene’s Technical Director, who provided this excellent technical explanation of elastomeric paint.
Elastomeric Coatings vs Resene X-200
“Acrylic polymers, the main binders in paint, are formed by reacting (or co-reacting) with a series of liquid chemicals called monomeric acrylic esters. The best known of these is a monomer referred to as MMA which, when polymerised with itself, forms the solid polymer known variously as Perspex; Plexiglass; Lucite etc. It is especially renowned for its extraordinary durability.
Poly MMA is too hard for use as an air dried paint binder so it must be softened in order to be able to form a film at ambient temperatures. To do this, MMA is co-polymerised with softer acrylic monomers to produce a wide range of polymers with varying degrees of softness.
The higher the amounts of MMA used the better the durability, the dirt pick up resistance, the block resistance and the barrier properties towards carbon dioxide. Low levels of MMA contribute the reverse of these attributes, but with the extra softness comes a significant degree of extensibility. Back in the day, this was used as a marketing gimmick and highly extensible, high build mastics were offered to the market. Very poor dirt pick-up coupled with poor durability saw the rapid demise of these.
The smarter resin manufacturers still saw some benefit in this extensibility and researched improvements in the technology, including a low level of self-crosslinking such that they would still stretch to a significant degree but the crosslinking gave added toughness and resilience, as well as the ability to slowly regain their original shape. These polymers could now be referred to as elastomeric rather than just extensible.
Although improved, durability was still relatively poor and the dirt pick-up was bad. This aspect was also addressed by an elegant technology which allowed a very thin skin to develop on the top of the coating which improved the dirt pick up as long as it (the thin skin) remained intact.
For all of the improvements, the major premise for the use of such coatings remains flawed. It is claimed that they can bridge propagating cracks! A propagating crack will always tear an adhering film that covers it because no film has the infinite extensibility needed to follow the crack. Because of the crosslinking, the best made elastomerics have a degree of tear resistance which can slow down the propagation of a tear through a film such that it is not immediately obvious – but you need a lot. 2,000 microns of mastic dry film is required to prevent the immediate tearing of the total film over a very small propagating crack of 1mm. The “notch” effect, however, will see the crack propagated through the full film in a relatively short time.
No building will ever be held together by a coat of paint! Where periodic extensions of a building do occur, the solution is best designed into the building or handled by elastomeric sealant joints The sealant joint is, quite properly, conservatively detailed with backing isolators; minimum joint widths and an allowance for no more than 25% joint movement.
High build coatings are best served by flexible (as against elastomeric) coatings which maximise durability, watertightness and low maintenance.
In our opinion, where a building is flexing due to height or design, an elastomeric may be the right choice. However, for most residential monolithic, textured, stucco or block houses Resene X-200 is by far the best choice. It has all the attributes required to fill cracks in the plaster and provide protection against water ingress along with inherent durability. In the event of cracks reappearing Resene X-200 can be rolled directly into the crack to fill it. It is also a lot cheaper as it has a higher coverage rate (m2/litre) than elastomeric paints.
For more on Resene X-200, check out this blog.