Before Bob Ross, there was William "Bill" Alexander (1915–1997), who also had a painting show on PBS television stations. "The Magic of Oil Painting" ran from 1974 through 1982 and Alexander was actually Ross' mentor.
Alexander was a German painter who specialized in teaching. His show was one of the first on television to teach people how to paint with oils and his series was a big success. Alexander's technique was called the wet-on-wet method, the same one that Ross is famous for demonstrating.
The secret to Alexander's majestic landscape paintings was an oil-based, white mixture that he called "Magic White." He would coat each canvas with a very thin coat of this before he began a painting.
"Magic White" is, essentially, white pigment in linseed oil that is mixed to the consistency of cream. Generically, some artists call it "fluid white." It's a trick that has been used by painters for centuries and allows for perfectly blended, smooth oils. It also cuts the amount of time it takes to create a painting because you can blend colors directly on the canvas rather than your palette.
All of this played into Alexander's signature style, which he taught to Ross, Robert Warren, and millions of students on TV.
There was always a bit of a rivalry between Ross and Alexander, but Ross became the better known of the two. While Alexander developed and sold Magic White, Ross also marketed his own product and branded it as Liquid White. However, you can recreate the same thing by yourself using basic paint materials that you probably already have.
Magic White is a clear, fluid base coat for oils. All you need to do is dilute titanium white with linseed oil. Mix these together until you get a creamy consistency. Some artists choose to mix equal parts of linseed oil and Turpenoid (or turpentine) to create this homemade medium.
Everyone has their own recipe and it's what works well for their personal technique. Experiment with these two or three ingredients until you get one that works best for you. Be sure to take notes as you develop the formula so you can recreate it later. In the end, you can save a significant amount of money and get the same effect as Magic White or Liquid White.
If you have not used Magic White or a similar underlayer product before, a few tips will certainly help in your painting. You can also view Alexander's video tutorials, which are archived on AlexanderArt.com.