Skinning in paint
Resene has done extensive research on the causes of skinning in paint and has identified that as the temperature starts to increase outside the pail, the air between the lid and the bulk paint will heat up quicker than the bulk paint. The paint on the lid will also heat up quicker than the bulk paint because it is not as well insulated. As the paint on the lid starts to warm up, water will leave this surface and condense on the surface of the bulk paint. As the paint on the lid loses moisture, it will start to dry, forming a film. This film of dry paint on the lid is referred to as skinning.

What we now know is that it is not the temperature per se that causes the skinning effect, it is the change in temperature that causes the problems. Even a change in temperature of 10°C can be enough to cause skinning. The more rapid the temperature change, the more likely skinning will occur.

This research work culminated in a modified pail lid to reduce skinning in Resene pails. This modified lid retains a thicker layer of paint on the lid to reduce skinning. The lid is textured, which increases the surface area of the lid, thus increasing the amount of paint which remains on the lid. The more paint held on the lid, the less likely it is to skin.

Preventing the conditions that cause skinning is still the best way to solve skinning. A large proportion of the skinning issues are caused after the paint leaves Resene and is due to the way the customer stores the paint. Paint is best stored at a constant temperature of around 15-20°C out of direct sunlight. Differences in storage and handling methods is why some contractors in the same regions have more skinning problems than others. If they store large volumes of paint in their van in the heat of the sun and then the temperature drops rapidly overnight, skinning of the paint is likely.

Refer also to an extract from Tony Van Dyk's SCANZ presentation 1999.