What happens if you try to transfer an image with the protective film still attached?

Sublimators often try to press a product with the protective film still attached. If you try to transfer an image through the film and onto the product, this causes bubbling, which leaves a textured-like feel or look or causes the protective paper to adhere to the product. Not all products have this coating, so the best way to discover whether a product has the coating or not is to carefully hold it under the heat press – the film should wrinkle, making it easier to remove.

-David Gross, Condé