ArcFlat weld blocks are manufactured by pouring molten nodular cast iron into a sand mold. The casting is then removed from the mold and precision CNC machined on all five fixturing surfaces. Great effort is made to minimize the risk of voids/porosity during the casting process. However, porosity does sometimes occur and can be revealed during the machining process. When this happens, a void repair is performed followed by either hand sanding or re-machining. Below are a few pictures showing what you might expect on your weld block for a casting void repair.
Casting void repairs are cosmetic, they will not affect the performance of your ArcFlat weld block. Please be aware that your ArcFlat blocks might have this defect, it is considered normal.