There are two very useful tricks to getting good results when French polishing.
The first is to achieve a one-foot-or-less “comet’s tail” trailing the pad as you move it over the surface—as shown in the accompanying picture. This tail is made by the alcohol in the French-polishing solution evaporating through the oil (also in the solution). If the tail is too long, the pad is too wet, and you’ll probably damage the surface. When the tail gets really short, just a couple of inches, this is the signal that you need to add more shellac, alcohol and oil (I use mineral oil) to the pad.
The second is to slowly reduce to zero the ratio of shellac to denatured-alcohol solvent as you near the end. There is still shellac in your pad, so you will still be depositing some shellac on the surface, but your “rag tracks,” which are the rag equivalent of brush marks, will be reduced to almost nothing.
I find it helpful to have two containers, one with two-pound-cut shellac and the other with denatured alcohol. I begin the French polishing by applying a little from each to the pad, together with a couple of finger dabs of mineral oil. As I progress, I reduce the amount of shellac and increase the amount of alcohol, until I’m adding only alcohol.