Specially formulated mediums make possible the wet-on-wet painting style. Currently available from 2 companies there are some differences.

Bob Ross' Liquid White Oil Paint (arrow), Liquid Clear Oil Paint and Liquid Black Oil Paint are fairly thick.

Alexander Art Company's Magic White, Magic Clear and Black Medium are fairly thin.

Step One First you MUST make sure your medium is well mixed. Shaking does not always result in a complete mix. It's best to stir the medium. If the medium is not well mixed you may wind up with too much thinner on your canvas. If this happens wipe your canvas clean and start over. Step Two How much medium? For an 18x24 canvas dip your large brush about 1 inch into your container. For a 16x20 canvas about 3/4 inch should do well. Step Three Apply the medium to the canvas. First I suggest you lay your canvas flat on the table. There are several strokes that can be used. I prefer a circular motion, working the medium into the canvas without the bristles leaving the canvas. An "X" stroke is sometimes used. Whichever method you use be careful when you get to the edges of the canvas. If you're going to "flip" medium onto your surroundings it will happen here. Step Four Testing the coverage. Gently tap the canvas with your fingertip. When correctly applied, the medium will coat only the ridges of your finger, like a fingerprint. If there is too little the lines of your fingerprint will appear broken. If there is too much, you fingerprint will be blurred. Test your canvas in several places to ensure even coverage. Remove any excess medium with a paper towel. Without reloading your brush redo your canvas to smooth and spread the remaining medium. Recheck the application.

A second way to check the canvas is to look for any "brush bristle" marks. If the medium is heavy enough for your brush to leave marks then you have too much. The above works with white and black medium. If you're using the clear medium put it on the canvas then wipe it off with a clean paper towel. Clear medium must be applied very, very, very thin. Okay, you're ready to start painting. Don't wait too long. The medium is slow drying but it won't stay wet forever.