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Tips for painters to survive summer from MPAV

Master Painters Association

Scorching summer a test for painters and their products – written by Master Painters Association – Victoria/Tasmania General Manager Jodie Rebbecchi

One of the most common problems, especially during warmer months, is blistering of acrylic paint on exterior wooden surfaces. Incorrect preparation of surfaces prior to painting can cause major problems which can be expensive to rectify.

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A common scenario is when a painter decides to paint a dull but usually sound surface, he/she sands the surface and applies two coats of exterior acrylic, within days large blisters have begun to appear on the new paintwork – rising with the sun and diminishing at night. 

From a customer’s point of view, it’s easy to see why they blame the new coating for their problems. The painter has prepared the surface, properly followed the label instructions and now the customer has unsightly blisters all over their timber (what a disaster).

Let’s go back a few steps and look at why this may occur. There may be moisture within the substrate beneath the paint film which converts from a liquid into a vapour when the surface is heated by the sun. Or – at some stage – the timber was painted with enamel which over the years, has become very hard and brittle and it lacks flexibility. 

When the painted surface is heated by the sun, blisters occur because the moisture cannot escape.  Repainting exterior timber with a dark colour can also escalate blistering as dark colours absorb more heat. 

A new coat

The painting industry has always had problems with paint lifting on old previously painted weatherboards especially when painting over old enamel paint. Boards expand and contract, the paint does not. This movement literally shears the paint away from the wood.

Enamel develops millions of tiny cracks (usually not visible to the naked eye) and these allow excessive moisture to enter the board and when this board is painted and sealed with two fresh coats of paint, it traps moisture in the wood as soon as the sun and heat play on the surface. 

Prior to any preparation works or even at quoting stage, three things need to occur.  An adhesion test needs to be carried out and if the old paint starts to peel away then it should be completely removed and treated as a new surface to avoid the paint lifting off at a later time.

A moisture test also needs to be carried out and if the readings are in excess of 10 per cent, the timber needs time to be allowed to dry out.  Special attention needs to be paid to butt joins of timber when moisture testing. We also recommend that any buildings that were painted before 1970 have a lead test undertaken and if positive for lead, then lead paint removal procedures need to be adhered to.

The only way to totally avoid blistering and peeling issues, is to strip all timber back to a bare substrate and start again. All timber should be primed with an oil-based primer, filled where necessary, spot primed and followed by two coats of exterior acrylic paint. In many cases, the consumer does not want to pay for a complete removal as its expensive and labour intensive but it is in most cases unavoidable.

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