Some materials happen by default. You use them because you've always used them. In the case of primer I, and I suspect the majority of painters out there, default to acrylic gesso. There are actually sound reasons - it dries quickly, is affordable, can be used below both oils and acrylics, can be sanded down to an ultra-smooth surface (depending on what you've applied it to), is easy to clean up and has a useful amount of absorbency - but for many the actual reason will be that pre-made canvases come primed with it, so it's a familiar and natural choice when stretching your own or preparing boards.
Familiar and default do not necessarily mean best. I am now near the end of a painting I've been hacking away at for the last two months, and it will be the first painting I have completed with Roberson's Oil Primer.
This primer is perhaps the anti-gesso. Whiteness aside it is different in every way, from chemistry to application and to the way it takes paint. What makes it so different? Firstly, its an oil paint. It cannot be used beneath acrylic paint, clean up involves spirits or solvents and in the tin it smells and looks a lot like domestic gloss paint. Secondly, it has an extra ingredient: China clay - that's right, the stuff they used to dig up in Cornwall to make English porcelain.
In practice, this makes all the difference in the world. The runny consistency - like gloss paint whereas gesso is more like emulsion - makes it easy and quick and messy to apply but it gives you a choice - it lets you brush out most of your strokes but it doesn't self-level so it will retain your marks if you'd prefer. This decision will have implications further down the line for your painting so think hard! The marks it retains are more fluid and flowing than those retained by acrylic gesso and I have found they give an interesting surface well suited to woodland when applied chaotically. I suspect this would be just as useful for portraiture and figure work.
The other difference the clay makes is that the primer is far less absorbent than acrylic gesso especially when two coats are applied. This is the key difference and leads to a different way of working. Firstly, paint takes a lot longer to touch-dry. Not only does the drying time increase but, given a strong enough surface, brute force and solvent can knock back areas even weeks after they've been painted. Secondly, the almost watercolour techniques I use in the lower levels of a painting behave fundamentally differently. On gesso, as you've seen from my skies, the paint sinks in and gives a real softness, especially when worked over with a rag. On the oil primer, I'm more sparing with the technique due to the old "fat over lean" rule, but where I have used it it looks more like a stain than a wash. When worked over with a rag, you achieve a more mottled softness, and the brush marks in the primer give the colour a real complexity which is why it suits foliage so well. It also means it enlivens fairly flat areas of transparent colour as more paint is retained in the furrows. This applies to any glaze you add later on. Thirdly, thicker paint behaves very differently too. The primer is slippery where acrylic gesso is grippy - paint really can be pushed around and this can even get out of hand if your brush is too stiff. In fact you have far more options for manipulating paint but the paint is harder to control.
So there we have it, the anti-gesso. The two primers are truly worlds apart and experimentation is needed to see which suits you best. I will have both in my armoury from now on; acrylic gesso for the simpler, softer compositions, wide skies and wide spaces like the night paintings I am working on and oil primer for the more textured subjects like the avenue at Ankerwycke that is almost finished, for more complex compositions where I'll be changing my mind a lot and for subjects in front of a distinct and plain background.
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