I plan on painting on clayboard and having it framed and mounted to the wall of a building. Can you recommend an undercoating, such as gesso, for the front and back of the clayboard? What about a glaze or varnish after painting? Assume the worst case scenario as far as weather exposure, namely sun and humidity.
Exterior conditions, as you mentioned, present some concerns which need to be addressed as you begin working on this mural.

Humidity - Constant humidity is going to mean sealing the front, back and sides as well as possible in order to prevent mold growth, warping, delamination, support induced discoloration, and film whitening. Fortunately, many of these issues can be taken care of at the same time. Sunlight - Intense sunlight can break down certain materials fairly rapidly, so pigment selection and the use of varnish layers will be key to preventing the image from deteriorating.

The first thing you should do is seal the backside of the clayboard. We assume this is the Ampersand product, which is made with a very good grade of hardboard. Sealing the back is done differently than the front, as you don't really need to be concerned with yellowing. We suggest using the best quality exterior Polyurethane varnish for this. Apply a couple of coats to really seal the board. Next is the framing. Clayboard is fairly resistant to warping, but since you are in an extreme climate, it's best to frame the back and sides at this time as you would frame a building - with thick wood about 16" apart. Drill screw holes into the clayboard before you Poly-Urethane the backside, to eliminate any entry points of moisture. Use stainless steel wood screws and a quality wood glue to secure the clayboard in place. We assume you like the surface of the clayboard to paint on, so we would not suggest you need to apply more gesso or an acrylic medium in order to make this artwork more archival.

The choice of a GOLDEN paint line, such as Heavy Body, Fluid acrylics, etc., is up to you. Our paints are made for the professional artist with longevity in mind. Try to limit your palette to "Lightfastness I" colors. On each container, and in our Pigment Identification Chart, pigment lightfastness information is provided. We would also like to suggest applying the paints thickly and opaquely, instead of in washes or glazes. Even though the colors are lightfast, as fading occurs in a mural, underlying pigments keep the overall appearance similar. Thin layers of pigments have nothing to fall back on. Only the earth colors (Umbers, Ochres, Mars, Siennas, and Oxides) are so resistant to fading there is no real concern when used thinly. After the artwork has been applied and allowed to dry for at least a day if not a little longer, an isolation coat of acrylic medium and then the varnish should be applied.

You can refer to our Web site for information about this procedure, but we suggest using MSA Varnish, it has excellent UV protection, is non-yellowing, and resists cracking over time. It can also be removed for future conservation without harming the underlying artwork. Two coats are the minimum suggestion for exterior artwork.