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Whether you're using a new glass or one of your old favorites, it can accrue a layer of dust or dirt just sitting in storage. Make sure it's sparkly clean -- run it under soapy water and rinse it well. Allow it to adequately dry. You don't want to paint on glass that's streaky or wet. Remove any oil, dirt, soap film, or fingerprints that will mess up your paint work.  Let the glass sit to dry for about 7-10 minutes after rubbing. If you don't have rubbing alcohol, use white vinegar; it has similar cleaning properties. You want a solid 3/4" (2 cm) of your rim to be paint free. As some paint is toxic, it's important not to paint on any surface that could reach your mouth. What's more, the paint will be more likely to deteriorate if it meets drink or your lips. Make sure the tape is even. If it's not perfect, it's super easy to adjust. Use masking or painter's tape for best results.

Summary:
Wash the glass well in warm, soapy water. Clean the surface of the wine or martini glass with rubbing alcohol and a paper towel. Place masking tape on the rim of your glass.