Varnishing in Summer

Water Based Varnishes
Put simply, water-based acrylic varnishes are made up of microscopic beads of acrylic dispersed in water. Acrylic is clear and water is clear; however when the two are put together they appear milky. This is due to the difference in refractive index between water and acrylic. When light rays pass through this mixture they are defracted or bent to such an extent as to appear white. When the water evaporates the "whiteness" goes with it. It follows that if water is trapped in the varnish it will remain milky or cloudy. Water can be trapped in the film of varnish in a number of ways: recoating a varnish before it is dry, application of a thick coat, or the top drying too quickly. Varnishing in summer sometimes means that you have to take the temperature conditions into consideration.

Recoating time: If a water-based varnish is recoated before it has had time to dry, it may go cloudy even if it appeared to be touch dry and clear. The water from the second coat will "re-wet" the first coat and, as described above, will cause cloudiness. Drying times may vary from varnish to varnish, with the thickness of the coat, and of course the climate. Follow the direction on the label; if in doubt leave to dry for at least 6 hours.

Thin coats: If the varnish is applied too quickly it can start to cure on the top while water is still trapped on the bottom of the film layer. As described above, this can lead to a cloudiness in the varnish film. To avoid this, apply the varnish on as thin as possible; do not flood the surface. Varnish the work in a position closer to vertical than horizontal. This is most important when varnishing work with high relief or 3-dimensional surfaces, which can catch the varnish, resulting in a milky pool. (But be careful to avoid runs in the varnish!)

Forced drying: Even if the varnish has been applied thinly but is allowed to dry too quickly, water can be trapped in the varnish film. To avoid this, DO NOT force dry the varnish. Hair dryers, heaters, etc, can cause the varnish to dry and cure on the surface, trapping water underneath. If heating is required, use indirect heat not above 35ºc.

Ambient temperature: In the days of the old masters varnishing was a very tricky business. The temperature and humidity had to be just right and had to remain that way while the work dried. Today's varnishes are much more forgiving; however they too have certain limitations. As with artificial heating, it is strongly advisable to varnish in a temperature range within 5º-35ºc with a medium to low level of humidity. The minimum "film-forming" temperature is approx 5º. Minimum film-forming temperature is the lowest temperature at which the varnish will bind together, forming the tough interlocked coat that is desired. If the temperature climbs above approx 40ºc, the water may tend to flash off or evaporate far too quickly leaving the top of the film of varnish dry and cured, trapping the water underneath and leading to cloudiness. Extremes of humidity should also be avoided. In especially humid conditions (Auckland in January for example) the water may not evaporate fast enough and be trapped inside the varnish. However humidity is not as much a concern as the temperature and will only present a problem in extreme conditions.

For the most part these problems should never arise. However it has been discussed more out of interest for those artists working in abnormal conditions.



~:-)avi)