After blasting a bottle, you may decide to paint-fill it for that extra personalization. That means you need to decide which painting method to employ. If the individual elements of the design are easily separated, you might consider spraying your paint on and masking between layers of color.
It will come down to personal preference if you spray paint with the stencil on or off. Leaving the stencil on contains the spread of the paint but will require the removal of the stencil relatively soon after painting. The time frame for this is determined by the drying time of the paint you use. You do not want to let the paint cure before stencil removal as it will form a bond with the stencil and tear upon removal, serrating the edges. This will not give you a clean edge around your shapes.
When you remove the stencil prior to painting, you need to re-tape around the areas to be painted to somewhat confine the spray. Then, after the painting is completed, you need to wait until the paint is dry to the touch before using a razor blade to remove the paint from the un-blasted portions of the bottles. It’s important to frequently get a new blade to clean the surface. Most scratches are created by using the same blade for a long time, which, through paint buildup on the blade, do not lay flat on the glass surface and will scratch with a corner of the blade when pressure is applied while cleaning.
You can use good quality spray paints, such as Krylon, Rustoleum, or Valspar, or, if you want to use an airbrush to achieve gradations, you would use a regular airbrush with water-based airbrush paints. This requires that you know how to use an airbrush, which takes a little time to master. The drawback to bottles of spray paint is that the nozzles often clog up before you have a chance to use all the paint.
Both of these methods require air ventilation and/or exhaust systems for painting indoors.
—Ruth Dobbins, EtchMaster