Oil Painting Trouble-Shooting
Today''s 20th century oil paints have major advantages over the paints used by the "Masters," most notably their permanence or lightfastedness. Some colors are still not any more permanent than those used centuries ago due to a  lack of permanent pigments in certain color ranges.

Color and Overworking
Oil paint is probably the most forgiving medium because it can easily be reworked, hours or days later. Its colors, unlike acrylics, remain constant as they dry so that the color that you mixed on your palette will be the same on your finished painting. However, overworking a wet surface will muddy the color.

Brushes
Always use natural bristle brushes. Synthetic brushes don''''t hold the oil in their bristles and may react with the oil. Contrary to popular misunderstanding, turpentine shouldn''''t be used to clean brushes. Use mineral spirits or as an alternative, if you''''re allergic to solvents, work baby oil into the bristles and then clean with soap, such as dishwashing liquid, and water.

Solvents
Personally, I love the smell of turpentine and other solvents . . . but the use of odorous, potentially hazardous solvents is often cited by painters as their reason for preferring other media. Eco-House, Inc. manufactures a complete line of natural chemistry varnishes and solvents for artists, eliminating any health hazards normally associated with oil paints. Water-soluble oil paints and mediums eliminate the need for traditional mediums and solvents completely. Two of the major paint manufacturers offering these innovative paints and tips on using them effectively are Grumbacher Max 2 Oil Paints and Winsor & Newton Artisan Colours.

Varnishes
Varnishing your painting will protect and enhance your painting. Even though a painting feels dry to the touch, it is still drying beneath the surface. Wait six months before varnishing to ensure that the painting is thoroughly dry and to prevent surface cracking.Always apply varnishes in an atmosphere of low humidity to avoid blooming, or a white haze or film,  on the painting which can appear after the varnish is dry.

Oiling Out Before Varnishing
If your painting''''s overall appearance or certain areas are dull before varnishing, you can regain the sheen with a process called oiling out. When the painting is thoroughly dry, rub a general purpose medium or a mixture of thickened linseed oil diluted with 50% mineral spirits or a commercially produced retouch varnish into the dull areas. Wipe off any excess. Repeat the process until the painting has an even sheen. Note that applying varnish to a painting with dull areas, without oiling out, will not solve the problem. The duller areas of the painting will show through the varnish and additional coats of varnish will not  solve the problem of an uneven sheen.

Dull areas of a painting are caused because the oil in the paint has sunk to the next layer of the painting. The most common causes of sinking is the use of too much solvent and too little medium - mixing skills can only be perfected with experience but starting with equal parts is a good guideline. Another common cause is an overly-absorbent painting surface. This cure is simple -  apply several coats of gesso, sanding in between each coat, until the canvas is sealed. Pre-gessoed canvases are convenient but their superficial gesso coat makes them more absorbent than a canvas that has been prepared with several coats of gesso. Canvas boards, often used by beginner painters because of their inexpensive price, are notorious for creating dull areas in a painting - oils sink unavoidably into the mounting board. Canvas selection and its preparation may be more expensive than prepared surfaces but can, in the long run, prove as economical and are certainly less frustrating.

Drying Rates
The differences in drying times, varying from 2-12 days, occur because each pigment reacts differently when mixed with oil. Modern paints are formulated to optimize overall drying rates, decreasing the problem of slow-drying underlayers.

The following guidelines provide estimates of the drying times for standard colors. Note that these are approximate times and will vary depending on the manufacturer and the thickness of the paint.

Fast Drying (2 days): Cobalt Blues, Prussian Blues, Raw Sienna, Umbers, Ultramarine Blues, Ochres, Burnt Sienna, Mars colors, Ivory Black, Lamp Black, Titanium White, Zinc White

Medium Drying (5 days): Permanent Alizarin Crimson, Cadiums, Cobalt Violets and Greens

Slow Drying (5+ days): Yellows, Oranges, Alizarin Crimson

Fast drying mediums are available which will literally cut traditional drying times in half.