Dye-Penetrant Testing this test can be applied to any type of metal but is used best for non-porous, non-ferromagnetic metals. Dye Penetrant is only a surface indication test meaning that if the indication is in the weld or under the weld it will not be seen by this test, but MT cannot be done on aluminum or stainless steel therefore this test is a better method for these metals. For PT testing the technician applies a dye to the metal/weld and lets it sit. This is to insure any indications will have been penetrated by the dye. The technician will then clean the weld thoroughly and apply a developer to the inspected area. The developer will extract the dye from any indications showing where there could possibly be a failed indication.