What are Porcelain Onlays? At Southview we use the CEREC machine to mill porcelain onlays for teeth. These are used to restore teeth where cavities or other tooth damage has compromised too much of the tooth for a traditional filling to effectively fix the tooth. Therefore, it’s a great way to preserve as much of the patient’s existing healthy tooth enamel as possible, which is ideal for maintaining a strong, long-lasting and natural mouth.