Two-Varnish Finish System
This approach to finishing off an acrylic work is universally recommended by conservators the world over. More and more, the larger galleries are requiring their artists to use it.
Firstly, one or two coats of water-based acrylic varnish such as MM 7 Polymer Gloss Varnish & Gloss Medium is applied to the work then, when thoroughly dry, a coat of removable solvent-based varnish such as MM 14 Final Varnish Gloss Finish is used to finish off the work. Using a removable varnish allows this varnish to be stripped off, taking with it years of accumulated airborne rubbish, then a fresh coat applied. Because both of these varnishes are acrylic, they have the flexibility and elasticity to withstand the constant movement and expansion and contraction of most substrates. Therefore, these varnishes will not crack or show signs of stress as they age, neither will they yellow with age and they will help to protect the work from the destructive effects of ultraviolet light.
Step 1. Water-Based Varnish : A water-based acrylic varnish (such as MM7 Polymer Gloss Varnish and Gloss Medium or MM6 Polymer Matt Varnish) affords the work it covers substantial resistance to physical and chemical attack; however, this varnish is not completely impervious. Water-based varnishes dry by evaporation, thus the evaporating water can leave microscopic capillary holes in the varnish, which allow the paint to breath and cure. Allowing the paint to cure is of benefit; however, the small holes which are left behind have two potential drawbacks.
The first is that of allowing water back under the surface of the varnish. This will not affect the paint or varnish chemically (as long as both the paint and varnish have cured). However, when the acrylic and water are combined, their differences in refractive index mean they may look translucent or white. This may, in turn, produce cloudiness in the varnish over the work. This is only likely to occur when the work is literally soaking in water for a great length of time. Removing the water and applying an indirect warm dry heat can usually reverse this cloudiness (providing the varnish was cured prior to exposure to the water).
The second problem is that of dirt and grime catching in these holes. It is very easy for airborne rubbish to catch in the tiny holes which may, in tropical climates, lead to the increased chance of bacteria and mould growing in these areas.
However, in the majority of cases, the water-based varnish will be sufficient. This varnish alone is most certainly better protection for the paint than nothing at all.
Leave at least 48 hours for the water-based varnish to dry before applying Step 2.
Step 2. Solvent-Based Varnish : It has been found that by applying a removable final finish varnish such as MM14 Final Varnish Gloss Finish (turps-based) or MM15 Matt Varnish (turps-based) after the water-based varnish, conservators are able to easily clean and repair the work. When the work is to be cleaned, the final varnish can be redissolved in mineral turpentine; the water-based varnish helps to protect the paint during the removal of the original final varnish. Then a new coat of final varnish is applied over the clean original water-based varnish.
This process is recommended for those artists seeking the best long-term results from their materials and most certainly recommended for exterior mural work.
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Here is the web site link for information on varnishing and the types of varnishes available.
http://www.matisse.com.au/pages/varnish.htm
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Getting back to Wattyl, I've used wattyl products for over 20 years. Great product. Easy to use, and more readily available then Dull coat which seems to go through periods of drought (hard to obtain)
regards