Before the coating process, aluminium must undertake a surface treatment in order to assure the adherence of the polymer powder to all the external faces of profiles. This process takes place in the pre-treatment tunnel and is divided by three stages. The stage of grease removal treats the surface of profiles in a way eliminates superficial oil, dirt and even some oxides. The phase of de-oxidation eliminates the alumina layer naturally formed on aluminium surfaces and prepares the surface for the phase of treatment with chromium. During this phase of treatment, a chromium coating is generated on the surface of aluminium; this last phase may be completely omitted if necessary.