Without proper precautions, small white dots in your finished product can be one of the most common sublimation issues. To prevent white dots, consider this recommendation:
Don’t peel the protective film on the panel until you are ready to sublimate. Many sublimators will remove the film and leave their panels for days (or even weeks!) before sublimating, giving dust and other potential for damage easy access to the panels. We recommend waiting to remove the protective film until you are ready to sublimate.
-Jory Hutchens, Unisub