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 light 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 so too 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 few ways: recoating a varnish before it is dry, application of a thick coat or the top drying too quickly.
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 varnish 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; if in doubt leave overnight.
Thin coats : If the varnish is applied too thickly, 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 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 works with high relief or 3-dimensional surfaces, which can catch the varnish resulting in a milky pool. Be careful to avoids runs in the varnish.
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