How to prepare a substrate ready for an artwork in oils or acrylic. Canvas wood, glass or cardboard.
You should approach every work of art as if it has the potential to become a 500 year old museum piece. You can never know if this will be true, but potential is one thing, and actual archival robustness is another.
It is certain that a poorly prepared medium will not last long, and therefore in this case, you can indeed be sure that your crumbling works will not be gracing the museums of the future, while your robust works still have a chance.
Let's get down to basics. All paint is fundamentally colored glue. This glue is bonded to a substrate like canvas, wood, cardboard, paper, glass and many others. There are two important things to consider. The first is adhesion, and the second is flexibility.
On adhesion, you will find that a particular glue will adhere to particular substrates well, and other will not. A good example. is cork. If you want to bond to cork, then you need a latex based primer. The latex will soak into the cork, and remain flexible. It will seal the cork and permit a second layer which might be contact adhesive. If you used contact adhesive directly then the cork would absorb the glue and prevent it from sticking.
You can't glue to dirty and dusty surfaces. You can't glue easily to smooth shiny surfaces. In the first case, the substrate is separated by dust which will eventually fall off, and in the second, the glue will simply peel away.
Dust therefore should be removed, and shiny surfaces need to be keyed. Keying means to introduce tiny scratches which makes a surface that will permit the next layer to grab on. This is why you should lightly sand between coats of paint on door frames and skirting etc.
When preparing a substrate for a work of art, be sure to remove contaminants like dust - and of course oils and water. Seal porous material, and key shiny material.
All materials expand and contract with temperature. Some expand more than others, and some are flexible. Those that are flexible will not crack or buckle during temperature change, and those that are brittle will break and buckle if they are pushed too far.
Some materials will expand uniformly as the temperature changes, and some may expand and contract more in one direction compared to the other. Many substrates are affected by moisture. A good example is that of wood. Green wood can shrink perhaps 10 or 20% across the grain as it looses moisture, and 4% along the grain. It depends on the type of wood for how big is the effect.
To cope with expansion due to moisture, it helps to acclimatize the material first, then seal it both sides and all edges. Acclimatization means that you must leave it in a similar environment to the expected long term storage until its dimensions stabilize. When you seal the substrate the moisture content will be locked in. This will make it more robust as humidity changes.
Especially in the case of wood, seal it front, back, top, bottom and sides. This contains the current moisture content.
On canvas, a traditional approach is to prime it with rabbit skin glue (RSG). This seeps into the fibers and protects the canvas from harmful effects of oil based paint. Following the RSG, many artists will apply perhaps five layers of gesso, sanding between coats. This gives a strong surface that interfaces well between the RSG-primed canvas and the painting layers.
Remember that oil paint never theoretically dries. It can take a year before an oil painting is dry enough to varnish if that's what you like to do. Thinned paint layers, and those using a medium like liquin dry faster than thick layers, and those mixed with linseed oil.
As oil paint dries it looses flexibility. Therefore two situations can cause an oil paint to eventually crack. The first happens when the substrate expands and/or contracts more than the limits of the paint's flexibility, and the second is when you paint thinned fast drying layers on top of thick slow drying layers.
The recommended process is to paint "Fat over lean" which means the thick flexible slow drying layers are applied on top of dry thin layers. When you do this the other way around, the thin layer shrinks and cracks over the thick layer.
Your substrate should therefore be stable to temperature and moisture. It also needs the correct primers and preparation to provide a good interface between the paint and the substrate.
In short, a primer sticks well to the substrate. An undercoat sticks well to the primer, and the paint sticks well to the undercoat. Therefore, the paint is ultimately well bonded to the substrate.
Acrylic paints are much more forgiving, but will still benefit from a well prepared canvas, wood, or cardboard.
If your paints have good permanence, and are applied correctly, a well prepared substrate will give your paintings the best chance of lasting hundreds of years and remain bright and free of cracks and flaking.
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