The convenience of oil painting is that the colors, even when thinned, cannot run into each other unless they are so fluid they run from the palette too. Thus, although the paints are put on as a more or less soft paste, they make no hard rims as does watercolor when the paper is too dry. And though here, as everywhere, it is much frowned upon to mix colors on the picture itself, the consistency and long hardening time of oily binders do allow a little working together of paints lying side by side.
When the work has taken shape from the form and color point of view it is worked up with stronger colors and lights until the last high light adds the finishing touch.
If you have done it well then the painting can be considered truly finished. In some places the tinted ground stands almost untouched, and the more neutral-colored ground work shines through the upper painting, though in places this may be so thick that not even a black ground would show through. Thick painting gives the surface a pleasing, tactile, plastic texture. Carried to extremes this manner of painting has led to painting with the knife.
The method described above is that used by the majority of painters since the beginning of Impressionism to the present day. Technically it is preferable to make one's own glue ground, rather than to use bought ones; the best are on hardboard with glued-on canvas. The texture of the woven cloth corresponds to impasto alia prima technique much better than a smooth board.
The surface should be protected with shellac sufficiently to prevent its absorbing the binder from the bought tube colors; for this produces both yellowing and the so-called "sinking in" of the colors. When this occurs the absorption of the binder allows the colors to harden without any sheen, and as this never occurs uniformly it gives a blotchy effect to the whole picture. The paint sinks again, however often these mat areas are repainted, unless the area is first allowed to dry thoroughly and then given a coat of varnish. This process, however, would bring an entirely unwanted binder into the painting.
A finishing varnish gives the surface a uniform sheen and protects the penetration of dust into the unevenness of the paint. It must not be put on too soon. A good varnish is 1:2 or 1:3 mastic essence with at least 2 per cent addition of wax.
You can also dissolve wax alone in turpentine with a small addition of mastic to give a paste which is just spreadable. This varnish is the surest protection against damp; it should be applied very thinly with a soft bristle brush. The drawback is that it remains soft for a long time, unless you expose the picture surface, after careful brushing, to warmth until the turpentine has evaporated. The surface begins to sweat, and some water evaporates along with the spirit of turpentine. Strong heat should never be used with oil because this browns it rapidly. Gentle warmth, from 140 to 160 degrees F., is unharmful.
Another reason why coating varnishes should be put on very thin is so that they will not arrest the oxidation of the oils.
It probably takes 60 to 100 years for a thick oil paint to be fully oxidized. This is when late crack formation sets in.
We can make one final comparison between watercolor and thick alia prima oil painting: both methods are technically easy to manage. Both require ideally that the work be completed at one attempt in front of the subject.
But whereas watercolor painting entails no technical risk to speak of, oil and resin oil painting are the most dubious of all methods. However, corrections are always possible with oil paints. They allow the artist to work as long as he wishes at a painting to gain the greatest effect of form and color. Any error can be rectified without aesthetic damage to the painting, or covered over, scraped out, and repainted, and even made pure white right at the end. On the other hand, a watercolor is ruined if it is corrected or lightened with white.
Both techniques have one good thing in common: they show hardly any difference between wet and dry colors, whereas all glue techniques and fresco paintings suffer from a very great alteration. Opaque oil paints and watercolors are thus the best techniques for the beginner and should both be practiced.
Lastly, we must consider a branch of alia prima technique much used at the present time in abstract painting: palette knife painting. The paint is not put on the canvas with a brush but with spatulae of the most varied sizes, even some small trowels like those used by builders for laying bricks. The paint is used unthinned, straight from the tube, as though it were putty or mortar.
A palette knife cannot at once give such precise form as a brush, and modeling is done after the paint is put on with the edge and point of the knife. The character of the picture surface thus painted is rather like a low relief. The shadows of the solid paint are part of the effect and often constitute the picture's only charm. Spiritual content is often in short supply when so much seeking for effect is involved.
For if anyone thinks he needs the texture created by the shadows for realizing his artistic concept, he can paint it in with thin paint, and probably give thereby a better idea of whether he has something significant to say or is simply covering the surface with a colored texture which is more or less all there is to be expressed. Many, however, go a step further: they model a low relief in gesso and cover it with thin paint, or they squeeze the paint straight from the tube onto the surface and develop great virtuosity in spinning out the ooze in the finest threads.
I will offer no opinion as to whether with methods of painting such as these there are other, hitherto unrealized ideas to be expressed. But it is certain that such a use of paint by the pound in this way implies a great lack of respect for it. Its charm as a substance can be thoroughly enjoyed only by using the whole range between softest glaze and a thin body color.
Again, there is the question of the technical dangers of putting on paint inches thick at a time. Only in lucky cases will the compact layers remain without huge splits until late crack formation occurs. Thereupon, when the thoroughly oxidized oil begins to "burn," enormous cracks will be accompanied by the shrinking up of lumps of paint, and much of it will fall off. Even were this not to happen, the inevitable coarse splits will ruin the intended effect of the picture - quite apart from the fact that the thicker the paint is, the more the oils yellow and brown.