-Mary in HI
Your question raises two good issues. Let's take the first one: how to thin tube oil paint. Typically oil paint is thinned with a combination of turpentine, damar varnish and linseed oil. Ralph Mayer in his renowned The Artist's Handbook recommends the following formula for general, all-around purposes:
Stand Oil - 1 fluid ounce
Damar Varnish - 1 fluid ounce
Pure Gum Turpentine - 5 fluid ounces
Cobalt Drier - about 15 drops
Some artists just use turpentine and linseed oil in a mixture of 3 parts oil to 1 part turpentine. Either one alone will also thin paint, but this is not recommended. Turpentine on its own makes the paint too watery. Linseed oil alone can, in time, cause the paint to wrinkle. An alternative might be a Winsor & Newton product called Liquin. Liquin is a non-yellowing medium for thinning oils and alkyds. It also speeds drying time. There are several Liquin products currently available. Liquin Fine Detail is the most fluid and may give you the low viscosity you are looking for.
I'm not convinced, however, that any of the mediums mentioned above will give you the desired fluidity without sacrificing opacity. You would have to experiment to find the right paint to medium ratio. You might find the right combination but if you plan on working big it could get very expensive. Which brings us to the second issue: why not use house paint?
The reason Kline used commercial enamel house paint is that he needed the fluidity for Action Painting. That same fluidity is difficult to achieve with tube oil paint especially in large quantities. Many fine artists like Jackson Pollock, Frank Stella, David Alfaro Siqueiros and, of course, Franz Kline used commercial paint. In 2007 Christie's auctioned Frank Stella's Carl Andre for $3,961,000. And in 2006 a new record was reportedly set with the sale of Pollock's No. 5, 1948 for $140 million (you don't get much finer than that).
|No. 5, 1948 by Jackson Pollock|
Oil based commercial enamel paint is still oil paint and it is sturdier than the fine art manufacturers would like you to believe. The truth is no paint lasts indefinitely. Visit any art museum and you will see deteriorating paintings. If any material helps you create better artwork - use it. Lastly, if you are concerned about archival quality and permanency, be happy in the knowledge that you are creating job security for future art conservators.
Thanks for a good, thought provoking question.
All the best,
P.S. Be aware that there are health and safety concerns in the handling of all oil paints, mediums and solvents. Scout around the internet for information on specific products. The manufacturers or suppliers should be able to provide that information.