Aluminium:
If painting on aluminium, it needs to be etch primed: the panel is
etched using acid to create a fine tooth so that the primer does not peel or crack.
Wooden Panels:
Priming wooden panels can make them twist or warp. Prime the edges first as these are the most absorbent. For priming, the best is to use a subframe to hold the panel in place while you prime, and leave it in there until it is completely dry. If you are not too concerned, just paint a large cross on the reverse from corner to corner to counteract the worst effects of warping.
Gesso:
The support. Wood. Oak, Poplar, Pear, Holly are my favourites. Costly. So, old abandoned furniture is a good source. Also get off-cuts from specialist timber merchants and look in skips. Roughly clean it up.
Sand it. Size it: Rabbit skin glue: 1 part glue to 12 parts water; one thorough coat.
Gesso – Hard method:
Whiting: readily available and inexpensive (builders’ merchants.) Make up the size. 1 part glue to 9 parts water. This should be a stiff jelly when cold. Place the whiting in a metal bucket.
Reheat the size, as hot as possible without boiling it. Add this to the whiting, stirring slowly with a wooden spoon until it is free from lumps and is the consistency of single cream. Rigorous stirring will aerate the mixture and cause problems when drying. Apply this evenly to the support with a flat brush. Allow to dry.
Check the dry surface carefully to make sure there are no pinholes or cracks. These will remain always, no matter how many times it is recoated and must be avoided.
Repeat this at least 4 times with each coat of gesso slightly weaker than the previous one (this is almost a naturally occurring process as the size in the mixture will weaken upon reheating). I often apply many, many more coats. Don’t worry too much about brush marks as these will be removed when sanding back the surface.
Sand it: I use wet and dry. Starting with 180 grade, ending with 1000 grade. Until the surface is perfectly white and as smooth and hard as glass. Excellent luminosity can be achieved by this method with less immediate absorbency of the paint.
Gesso – Soft method:
Same as above but with a weaker size. 1 part size to 12 or less parts water. Diminished luminosity, very high absorbency of paint.