Testing the thickness of the paint
Using your smaller brush, load the paint into it, and practice applying black lines on a clear piece of scrap glass. If the paint seems too thick, add a very small amount of water.
Paint that does not stick
If the paint seems to bubble up or not adhere to portions of the glass, it is likely the glass is not clean. You can wash the glass, or simply add a little more water to the paint already on the glass and rub the paint over the glass with your finger or a small piece of paper towel. This will remove any dirt or film of oils on the glass.
Testing the amount of gum arabic
Allow your painted lines to completely dry. You will notice the dry paint has a chalky, opaque quality. Test your paint lines by rubbing a finger across a line. If it easily rubs off like powder, you need a bit more gum arabic. If you can't budge it and it seems hard and crusty, you have much too much gum in the mixture. Adjust the mixtures accordingly.
Inspecting the fired glass
When the glass is fired in the kiln, the paint (which is made of ground glass and various ground pigments) fuses with the glass. Too much gum in the mix, and the paint may bubble, sit on the surface, or do a few other ugly and unprofessional tricks to embarrass you. You have no choice but to start over with a new piece of glass. Getting the amount of gum arable right is crucial to the process.
If the lines are not consistent in colour depth, you can trace over them and fire again. This will darken the lines to a consistent level of colour.
Practice your tracing. This part of the art is like calligraphy - half the battle is learning to use your tool, the tracing brush, in one or two confident strokes. The quality of your trace line tells the world whether you are an amateur or an accomplished artist! You might even decide this is the only glass painting technique you will ever use. And you would be in good company. A good deal of Gothic stained glass relied solely on tracery for its embellishment and to good effect.