Oil Painting Methods
I produce all of my oil paintings on linen canvas primed only with oil-based primer - I never paint on surfaces primed with acrylic. I've done many paintings on Artfix L64C, and have liked that well enough, although my favorite canvas at the moment is Claessens #13 double-primed. Currently, I use Old Holland paint almost exclusively.
As for painting tools, most of the work gets done with the use of ordinary bristle brushes - I use filberts and flats most often. In addition to the bristle brushes, I also use a selection of softer brushes to carefully smooth the paint surface, primarily because I don't like the distracting effects of glare that so often occur on the ridges of brush marks typically found in thicker applications of paint. I've lately been using brushes made by Silver, specifically their Bristlon (stiff bristle) and Silverwhite (soft, synthetic sable) series - I've found that these hold up quite well despite repeated use over time, and they don't fall apart during cleaning.
I'm a very tidy painter, and I thoroughly clean my palette and brushes every four hours or so. I like to return to a clean, organized palette after taking a break.
Before beginning a painting, I typically tone my canvas completely with a very thin layer of some neutral color, often a combination of ultramarine blue with some earth-tone like burnt sienna or transparent oxide-red. This process begins by wetting the entire canvas surface with linseed oil to render the surface more receptive to the spreading of paint. All of the excess oil is wiped off until only the thinnest film of oil remains - this is sufficient to make the canvas quite slick, and on this wet surface I can easily spread paint with nothing more than a folded paper towel - I prefer the blue "shop towels," which are a bit more durable for the job and leave far less lint debris in the paint. I'll often begin drawing directly into this wet layer, using the towel as a drawing tool instead of a brush - an underlying, monochrome drawing can be established quite quickly in this manner, without even getting a brush dirty. Highlights can be rendered simply by wiping the paint away to reveal the white canvas below. At this stage, all of the paint remains very thin and transparent.
In this manner, I'll often complete a fairly elaborate, well-considered under-drawing before beginning to work with color at all, since I like to know that the primary drawing issues (composition, proportion, symmetry) have all been resolved in the early stages of the work. This leaves me free to concern myself entirely with color as the painting develops, and it always makes the work proceed much faster.
To thin my paint when necessary, I may lightly dip my brush into odorless mineral spirits (typically, Gamblin's "Gamsol" product), or I may use a small amount of refined linseed oil, or sometimes a combination of the two. I never use turpentine as a solvent, due to the harmful effects of its fumes - I always get terrible headaches from turpentine (in contrast, Gamblin's odorless mineral spirits seem to produce no ill effects for me). There is a balance to be achieved in the thinning of paint, since free-flowing paint is desirable, but too much thinning can be destructive to the integrity and binding power of the paint. The addition of too much oil to the painting's surface can also lead to a yellowing of the work over time, so I strive to avoid the excessive use of oil as a thinning medium. I never use any of the thick, resinous mediums that are widely available, since these tree-derived resins are known to contribute to cracking and yellowing of the paint film.
I've been trending toward a more limited palette of colors during the past couple of years, and now rarely use more than five colors (including white) in the production of any painting. Cadmium Yellow and Cadmium Red generally suffice for the warm side of my palette, while Ultramarine Blue and Manganese Blue are usually the source of my cool colors. I'll use either Titanium White or Lead White, depending on my needs at the moment, since these whites have very different properties. The Titanium is very opaque, and has strong covering power, but I find that it can darken in value slightly while drying, and therefore is less reliable for over-painting when an exact color match is required. When in need of a transparent white, or one that will produce a more reliable color match for over-painting, I use Lead White.
Speaking of over-painting, virtually every square inch of my paintings receives at least two applications of paint, since I'm a perfectionist and in search of a very specific result. The first pass in any area rarely hits the mark quite accurately, and requires further attention after the first layer has dried. There is a long-standing rule in painting, generally expressed as "fat over lean," which simply means that the initial, underlying layers in a painting should contain less oil than the final layers applied on top. The problem results from a disparity in the rate of expansion and contraction between the layers. "Fat" or oily layers are more likely to expand and contract during the drying process or in response to changes in atmospheric conditions - the more oil, the more lateral movement of the paint film. If the lower layers are more oily and mobile than those on top, they can pull the more brittle top layers to pieces, resulting in cracking. As a result, it's necessary to ensure that the greatest percentage of oil is to be found in the top surfaces of the picture.
In applying one paint layer on top of another, I've found that the underlying layer often develops a glassy surface resistant to oil - by applying linseed oil to such a surface, the oil can be seen to bead up, rather than coating the surface evenly. This resistance would logically seem to undermine the chances of a reliable bonding between the old and new paint layers, and can definitely interfere with the achievement of subtle effects while painting, so I attempt to prepare the dried surfaces before painting over them.
I do this by mixing refined linseed oil with "Bon Ami" powder, a common, gentle abrasive used for household cleaning, made entirely from harmless calcium carbonate. A small amount of powder is all that's required to perform the task. The powder is mixed thoroughly into a puddle of oil until the abrasive is reduced to tiny, invisible particles. I then use a bristle brush, loaded with the powder/oil mixture, to carefully scour the surface of the painting in the area to be retouched.
Although the oil mixture initially beads up, it will soon begin to cling to the surface quite evenly, a sign that the powder is abrading the paint surface. If this is done gently, and without too much powder in the solution, the surface can be altered without any visible harm to the paint layer (for instance, grinding away too much paint or otherwise changing its appearance). I've found that trying to perform this same function with a solvent like mineral spirits will invariably cause visible damage to the underlying paint layer, which can be difficult to hide even with a great deal of overpainting, especially if one intends to apply transparent glazes. The Bon Ami powder is much more gentle, and has the added benefit of lacking noxious fumes. After the surface has been adequately prepared, I use a paper towel to rub away as much of the excess oil as possible. No residual trace of the powder can be detected, and the next application of paint will adhere to the preceding one without resistance.