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Preparing Panels for a Life Outdoors

6 thoughts on “Preparing Panels for a Life Outdoors”

  1. Re waterproofing panels for indoors. I saw the following post by Sarah. Is shellac a reliable surface to paint on then, if it gets brittle with age, isn’t the painting at risk? Might be better to skip waterproofing? Is it mostly to stop warping? Ditto is an aluminium coating instead of shellac going to last? Advise often is use artist products because commercial formulations can change recipes and aren’t intended to last so long. If the primer fails, the whole painting fails. Are there other alternatives that are likely to last longer? Do you then put gel or gac100 on top of that? Thanks

    “Shellac is often used by woodworkers as a preferred sealer for wood as it is not waterbased and so will not raise the grain, dries very quickly. However we would opt for a pigmented shellac – such as BIN’s Original – as being a better water moisture barrier and take that information from the Forrest Products laboratory, which are truly the experts when it comes to research into wood finishing and protection. You can find information about their recommendations in various places. Here is one link to a list that includes pigmented shellac, which rates surprisingly not all that far behind even a two part epoxy:


    Another document supporting pigmented shellac over clear can be found here – a much more technical document to be sure but will quote the relevant part from page 14:

    https://permanent.access.gpo.gov/websites/www.fpl.fs.fed.us/documnts/fplrp/fplrp462.pdf -(Note: MEE stands for Moisture Excluding Effectiveness. The higher the number the higher the percentage of blocking moisture.)

    ‘The low values of MEE14 for latex finishes stand in contrast to those of the shellac-, varnish-, or paint-based finishes that we evaluated. A white shellac (alcohol solvent) (finish 23) with an MEE14 of 73 percent for six coats was less effective than a pigmented flat shellac (also alcohol solvent) (finish 60) which had MEE14 of 83 percent. For each coat applied the MEE increase was greater for the white shellac than for the pigmented shellac. This greater increase in MEE with each successive finish coat for a nonpigmented versus pigmented finish was also observed with the gloss urethane varnish (finish 13) and the aluminum flake-pigmented varnish (finish 43). Increases in MEE for the paints (finishes 67 and 77) were similar to those for the pigmented varnish and shellac. Browne (4) has done an extensive study on the variations of MEE for a linseed oil paint according to the nature of the pigment. In general, pigmented finishes have much higher MEE than unpigmented finishes for any specific resin system.’

    As a last entry into the Forrest Products Laboratory Literature on this – and a bit more accessibly written – take a look at page 16-14 (Chapter 16 page 14) of their Wood handbook:


    Sarah Sands, Senior Technical Specialist, Golden Artist Colors

    • Hi Ralf –

      Thanks for your questions. Just so others know the context, the quote you share is from a reply I posted in a thread on MITRA (Materials Information and Technical Resources for Artists) which they can read here: https://www.artcons.udel.edu/mitra/forums/question?QID=379

      Let me reply to your questions individually:

      Is shellac a reliable surface to paint on then, if it gets brittle with age, isn’t the painting at risk?

      While it is true that shellac will yellow and grow brittle with age, as a very thin coating on an inflexible support like a panel, this should not cause a problem or endanger the artwork. In this way, it is not so different from dammar varnish, which is another natural resin that has also been used on panels, usually to lessen absorbency before applying a ground. But certainly shellac is not the only material that could be used, and if for some reason you would like a more substantial and physically durable coating, an alkyd primer could be substituted. Just keep in mind in all these cases we are suggesting these things as a way to lessen moisture penetration and not as a ground to paint on. While using these by themselves on the sides and back is fine, on the front we would still recommend the use of a ground.

      Might be better to skip waterproofing? Is it mostly to stop warping?

      Lessening the absorption of moisture, and mitigating against large swings in humidity, is a major concern with any and all wood products. Certainly, a large part of that is warping, but it is also related to lessening dimensional changes overall. The more a panel is sealed, the more physically stable it will be. So we think taking these steps are beneficial.

      Ditto is an aluminum coating instead of shellac going to last?

      The aluminum paint is in an alkyd binder and are used quite often in marine applications where a high level of moisture barrier and physical durability are important. However, given that artwork, especially ones being exhibited indoors, rarely if ever are undergoing that degree of exposure to moisture, we think you can skip this step for anything being shown and stored inside. But if you do apply it to the side and back of an indoor panel, we have every reason to believe it would be extremely long lasting given the track record of alkyds.

      Advise often is use artist products because commercial formulations can change recipes and aren’t intended to last so long. If the primer fails, the whole painting fails. Are there other alternatives that are likely to last longer?

      Our recommendations for sealing a panel are mostly related to the side and back of the panel, where we feel that commercial products do offer advantages and perform better as moisture barriers when compared to traditional oil-based primers and grounds, as well as acrylic ones. Those artist coatings simply are not very effective in blocking moisture. on the front of the painting, however, one should certainly follow up any application of a shellac or alkyd primer with a ground formulated for artist use as this will provide a whiter surface and with a degree of tooth and absorbency optimized for the needs of the artist. And if you remain hesitant to use these commercial products on the face of the piece, then simply apply them to the sides and back and use an acrylic or oil ground made for artists on the front.

      As for commercial versus artist materials and longevity, we think one needs to take that guidance with some perspective. nearly all the resins, oils, pigments, and supports (including canvas) that artists use were originally made for commercial applications and only later are selected by art material manufacturers as being appropriate for artist use. But the fact remains, almost everything we as manufacturers work with was made for some other use or market. So it is impossible to seal oneself off completely from commercial products. And indeed, there are many areas where artist materials simply would not the best or most durable choice. For example, direct to metal, glass, cement and moisture resistant wood primers are all areas where commercial materials will often be better suited than alternative ones made for artist use. This is particularly true if the work will be outside, where maximum physical durability and adhesion in a wide range of environments are critical. Indoors there is much more latitude and quite often – but not always – there is a suitable artist material that can be used instead. Where art materials should have a preference are the more immediate grounds, paints, and mediums that go into the actual artwork, as well as removable, protective varnishes.

      Do you then put gel or gac100 on top of that?

      We do not see any advantage in applying these on top of a shellac or alkyd primer being sued to seal the sides and back of a panel. If anything you would be adding a layer that is prone to dirt pick-up and will become soft and sticky in warm environments. On the front of a panel, we would recommend following up the shellac or alkyd primer with acrylic gesso or an artists’ oil or alkyd ground.

      Hope that helps!

  2. Where can I get the primers? I am working on an outdoor mural on a large scale plywood sheet and I am researching the best combination of materials to use. I am interested in using your paints but I am wondering what products work best with your paints as in primers, aluminum paint, latex, isolation coat and varnish. Thanks for your help!

    • Hello Cameryn,

      when buying commercial products for the first time, we recommend going to a good hardware store and getting advice there. In general building materials are competitively priced, therefore the price also reflects quality to some extent. As long as the instructions of the commercial primer indicate that it can be top-coated with ‘latex paint’, then our acrylic paints should be fine to use. Besides proper priming, the choice of wooden panel is also very important. We have seen and written about a common problem that occurs with plywood when used for the outdoors here: Surface Checking and Plywood, Is It a Concern? (https://justpaint.org/surface-checking-and-plywood-is-it-a-concern/)
      A better alternative to plywood would be MDO panels. If getting new panels is not an option, due to costs, you could adhere a sheet of polyester sail cloth onto the plywood. The plywood then would still likely check and deform, but the polyester canvas would give the painting some additional stability and prevent the cracking from transferring through the paint layers. We have not tested this specifically, but applying paper or canvas over wooden panels has been a common surface preparation technique for centuries.

      Please, also take a look at our mural tech sheet for a list of color we specifically recommend for the exterior: https://www.goldenpaints.com/technicalinfo/technicalinfo_murals

      Should you have more questions, feel free to email us at help@goldenpaints.com.

      Best of success with your mural!


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