miller,
Oils, Acrylics, enamels etc. all share a quality of being opaque particles of pigment suspended in a medium or binder of some sort. They lay down or adhere to the surface and are held there by the drying or curing of the suspension medium.
The difficult part with watercolors is that they are for the most part transparent (less pigment particles) and in this application, not as permanent. Their binder or suspension is really not for adhesion to hard, sealed surfaces. The mechanics of watercolors are that the pigment uses water for a suspension medium and that pigment becomes imbedded in the fibers of the paper or illustration board because of waterflow in and around those fibers. On a figure, there is no "fiber" other than the irregular surface created by minute particles of the primer that create a "tooth." While the color will sit on the surface of the primed object, it will not adhere and will easily come off over time. You will literaly be able to take a soft brush and remove the color.
The best bet would be a guache since it uses an organic binder that suspends more of the particles and will adhere (to a point) to a surface like primer. But only to a point. As the binder breaks down, so too will the adhesion characteristics of the guache. Making it fragile.
So....fiber or porosity (paper, wood, plaster, gesso, canvas, sand, stucco, spackle, leather, vellum) is needed for watercolors.
Smooth surfaces like plastic, resin, metal, glass, primers (acrylic, enamel) etc, can use paints that use something other than water to bind their color pigment to the surface.
More cool party trivia later......like, polar bears do not show up on infra red.
Happy New Year.
Mike
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"