Deciding what kind of paint to use on plastic usually leaves people staring blankly at the shelves in the craft store. There are so many factors to consider. It depends on the type of craft project, but the ultimate goal is durability. Paint flaking or chipping off of your plastic cups is not the look you want.
Acrylic paint is easy to work with as it works on a variety of surfaces, which can make it a good choice for plastic cups. Just as versatile as the many surfaces it covers, acrylic paint comes in different consistencies for different uses. If you are going to use an airbrush to spray the entire cup, there is a thinner, watery version of the paint. However, if you plan to paint designs on your cup, there is a thicker acrylic paint with a butter-like texture.
Acrylic paint has been around since the 1950s. A more recently developed product--designed for plastic--is called "Paint for Plastic." According to plaid.com, this durable paint adheres to plastic objects, such as cups, and will not crack or chip. It's water-resistant and non-toxic, so it's perfect for your cups if you plan to use them for drinking.
Plastic that is flexible has been a challenge to paint because it doesn't adhere very well. If your cups are flexible, the trick is to use primer first, before applying the paint, and then apply a sealer over your design for protection from chips and scratches. According to Michael's, an arts and crafts store, Paint for Plastic is designed to work on flexible plastic objects where other kinds of paints fail.
Use acrylic spray paint for larger items. Using a brush for larger items would be more labor-intensive and take much longer. The thicker consistency acrylic paint and the Paint for Plastic can be used on smaller surfaces with a brush to make decorative designs. Their thicker texture will make your designs pop.