As is evident, a semi-absorbent layer of size is necessary for both oil and gesso primings. Art shops will sell size of sufficient quality for this purpose; decorators’ “size” sold in DIY stores may not be good enough, and may not be rabbit-skin glue at all. Size is available in granule or in sheet form, often with instructions attached. The dry size is generally added to water, in the ratio 8 parts water to 1 part size and allowed to soak overnight. The resulting gel-like solution should be gently warmed, and NOT boiled, until it has the consistency of a thick liquid. At this point it might be desirable to strain out a few little lumps that might remain unliquefied, by quickly decanting the size through a membrane of plain muslin cloth into another vessel. The size should be sufficiently liquid to be evenly worked into the surface with a broad priming brush whilst still warm. On cooling, size will revert to a gel, and reheating cool size risks making it brittle on drying, so it is best to make a batch for one session only. Some instructions will recommend stronger mixes when applying it to more absorbent supports such as wood or MDF. Some artists consider that two coats are preferable. It is important to avoid the extremes of an insufficient and excessively absorbent layer, or one which has no absorbency at all, and presents a glassy and impermeable surface.