This can be a very tricky application. Many plastics, most notably acrylic, vinyl and polycarbonate, outgas. The result is that you label will show small bubbles that are the result of trapped gas. See the yellow label in the illustration shown here. Certain plastics, however, are less prone to outgassing (for example fiberglass). Another solution is to use a permeable label material, such as certain grades of reflective film.