Lately I have been having a problem with my oil washes. They look great when wet, but when they dry they leave a dusty, speckly, gritty, grainy deposit of color, rather than a thin outline of paint around the details. I am using both 502 oils, and have this problem using store bought mineral spirits, Mig Thinner for Washes, AKs Odorless thinner and White Spirit, and Humbrol enamel thinner. I am applying my washes to a gloss coat of future. Even as the wash is wet in the paint well, I can see the pigments separating from the fluid. Any ideas?

A couple of thoughts that might help:

First, the pigments in certain colors, like Payne's Gray, will separate out with occasional unintended or undesired results. The white pigments in Payne's Gray will, for instance, often leave distinctly whiter / light areas with other areas darker from the black pigments.

So, the problem could be inherent in the paint colors you're using. The different pigments might have different properties when in a solution that cause them to spread at different rates across the surface and separating into distinctly different colors than what you started with.

Second, I find it best to work in smaller areas using a "pin wash" technique rather than a general wash technique. I only apply the washes exactly where I want them, and then, before they dry, I use a slightly damp brush to manipulate and blend the edges of the wash. This eliminates tide marks and smoothes the transition between the wash and the surrounding area.

Finally, I don't apply my washes on a glossy surface. I use a flat or matt overspray after decaling and my washes and oil dot color modulation go over this. The matt surface is "grabbier" and holds the pigments in the wash in place better. Pre-wetting the area with a little thinner helps to control the intensity of the washes and allows them to be manipulated easier.

Pre-wetting also allows you to use washes that have more "body" and less thinner to start with. In fact, you can pre-wet and then apply very small amounts of neat oil paint and blend that out. However, I generally dip out a bit of the thinned oil paint that's about the consistency of honey and use that for my washes. I only use the really thin stuff when I need capillary action to draw the paint along a seam line or something.

HTH,