The painting for this post is a little more unusual. It has been painted on Arches oil paper, a fairly new product that looks very much like watercolour paper but has been specially treated to handle oil paint.
Normally, the oils and turps will eventually rot untreated paper (that is, a sheet of watercolour paper used "as is"). There is nothing wrong in using such paper for oil-sketching but you have to just remember that it may have quite a short life-span.
Arches oil-paper takes a little bit of getting used to; this is only the second time I've tried it. I find the surface to be fairly absorbent; it pulls moisture from the brush as you paint over the surface, causing the brush to "run out" of paint quite quickly. It does, though, permit turpsy washes to be put down, rather like a watercolour wash; very useful for blocking in the main components of a picture. Washes can be scrubbed around, too, the paper is strong and able to put up with some rougher treatment.
Once a certain amount of paint has been laid on, it becomes easier to add it with a painting knife. You can push the paint around and scrape it off with very few problems. In the case of this painting, I started to add the light parts of the clouds on top of the darker areas with a small knife. The bright ochre-orange foreground hillside was worked only with a small knife. It was easy to push the paint around on such a smooth surface; and the marks created did not sink down or disappear.
I think the paper tends to give a more matt finish to the painted image, than you would get on canvas, and certainly more so than on a gessoboard. However, I find this interesting because there is some resemblance to the matt appearance of a pastel painting.
Finally, how to frame something on oil-paper. Contrary to some beliefs, you don't HAVE to frame it under glass, just because it is paper. There are a number of ways of getting the paper mounted onto stiff backing-boards (I'm still researching them); this then creates a panel, which can be framed just like any other oil, i.e without glass. I imagine that, once strengthened in this way, and maybe also with varnish on it, the picture will be as robust as any other canvasboard or ply panel.
I might consider painting the same subject twice; once on oil-paper for a matt finish; and then in pastels on my usual surface, to make a comparison.