In the end, it all comes down to footwear.

Practical for plein air painting as long as there aren’t deer ticks.

CT asks: I paint with water-soluble oils. I don’t know if this goes for regular oil paints, too, but I’m struck by the various textures and viscosities of different colors—from a cadmium yellow so thick it’s hard to get it out of the tube, to oily paints like the siennas. I know that you’re dealing with different pigments, so it takes different amount of oil to suspend them. But when you are trying to paint with them, how do you deal with the extremes? (Maybe you’ll tell me that that’s not a problem with “real” oil paint.)
Yes, water-miscible oils behave differently from regular oils. When severely thinned, water miscible paint tends to slip around like watercolor. When used straight from the tube, it tends to drag more than conventional oils. This means that the natural impact of viscosity range is somewhat exaggerated for you.
That range comes from the pigments themselves.  A paint’s opacity is directly related to its particle size (ergo its viscosity). The oldest pigments—the earths—tend to have large particles and be relatively heavy paints, since they’re basically just ground-up minerals. The 19th century pigments—most notably the cadmiums—tend to be moderately-large particles and so are moderately heavy. The 20th century transparent synthetic organic pigments generally tend to be high stain and more transparent.
There are a couple other factors involved. Making paint is an art in itself, and various manufacturers mill and mix pigments differently. Different brands of paint have radically different pigment loads, so the same color from two different makers may vary greatly in texture. Some paintmakers use driers, and sometimes paints sit for a long time before being sold, meaning you occasionally come up with a half-dried tube right from the store. Paints that stand (especially in high temperatures) can separate,  so the beginning of the tube is all oil and the end is stiff.
A sophisticated painter understands and works with the natural weights of pigments. This is especially crucial in watercolor, but it’s true for opaque media as well. This is, in fact, the fundamental trick of indirect painting, where a base painting of transparent earth tones is laid down and then painted into with opaque paints.
Cute, and about $8 from Old Navy, but ridiculous for plein air painting.

TG asks: “What kind of shoes do you recommend for plein airpainting?”
Next week I’ll be painting during a cocktail party fundraiser for the Association for the Blind and Visually Impaired.  I’m tentatively planning on wearing red patent-leather flats, a beaded skirt, silk blouse and pearls (with a smock, of course). But I admit that isn’t my typical painting garb.
There are two major issues with footwear: that you can tolerate being on your feet for several hours at a time, and that they be suitable for the environment in which you’re painting.
When working in an area without deer ticks, I favor sport sandals that can tolerate water. (I often find myself not just painting the river but slopping around in the river.)

But ticks (or black flies) mean you have to have a bug barrier of some kind, and the most effective one I know is clothing: long pants, socks and sneakers.
In the winter, I wear waterproof hiking boots and wool socks if I’m likely to get my shoes wet, or sneakers and wool socks if I’m not. Some painters carry a scrap of carpet on which to stand.
If you’re in an area with rough trails and you plan on backpacking your painting stuff up them, real hiking boots are in order. There is no agony like that of insufficient footwear on a rough trail, particularly if you’re packing any weight in a backpack.

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