Preparing Panels for a Life Outdoors

Most painters have a sense of how to prepare panels for oil and acrylic paintings when they are meant to be shown indoors, and we even published a couple of Just Paint articles on the topic:

Understanding Wood Supports for Art – A Brief History

Plywood as a Substrate for Painting

However for both Muralists and Sign Painters questions can quickly arise when preparing panels for a life outside, where rain and humidity are a constant threat. What follows are our best recommendations drawn from industry research and our own field experience over the decades.

The first step, before anything else, is making sure you use an appropriate plywood as not all of them are made for extreme changes outdoors. We would strongly recommend working with either a marine grade plywood or a type of plywood called MDO (Medium Density Overlay), often referred to as sign painter’s board. Good lumber supply stores should be able to help you select one that is appropriate.


To prepare the panels for outdoors you will need to seal them as much as possible from moisture, especially along the sides, where water will easily penetrate. Below are the steps we recommend:

1) Make sure the surface is clean and free of any dirt, wax or oils.

2) Lightly sand all smooth surfaces to assure the primer has good adhesion.

3) Apply a minimum of two coats of a high quality alkyd-based exterior wood primer to all surfaces, including the edges and both the face and back. Exterior alkyd primers are available from most good commercial paint stores.

4) After the above has fully dried, seal the edges with an aluminum paint made from aluminum flake or leaf. This has consistently been shown to be the most effective moisture barrier when comparing various coatings.1 Most major manufacturers such as Sherwin-Williams®, Benjamin Moore®, PPG®, and Rust-oleum® produce these, where they are commonly used for marine, metal, or automotive applications.

5) Finish by applying a high quality exterior latex primer to the front and back of the panel. This will give additional durability and assure having a nice, bright white ground to paint on.

Once the panels are fully dry, follow the guidelines provided in our Exterior Mural Tech Sheet, paying particular attention to the recommended list of colors:

Murals (Exterior)

Once the mural is finished, you should apply an Isolation Coat and two or more coats of our MSA Varnish. For information on these, please see our MSA Varnish and Isolation Coat Tech Sheets, as well as our Mural Resource Guide:

MSA Varnish (UVLS)

Isolation Coat

Murals Resource Guide

As always, if you have further questions, please contact our Materials and Applications Specialists by email at, or call 800-959-6543 / 607-847-6154

1 See Williams, R. Sam. “Chapter 16: Finishing of Wood”, Wood Handbook, Madison, WI, Forrest Products Laboratory, pp. 13- 15.

3 Responses to Preparing Panels for a Life Outdoors

  1. ralf r May 18, 2018 at 5:25 am #

    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: -(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

    • Sarah Sands May 18, 2018 at 12:26 pm #

      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:

      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!


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