I rarely use a spot weld cutter anymore. I just grind through the spot weld on the panel that I'm not saving. One of these works well : https://www.tooldeals4u.com/product/db18100/ belt grinder. pricey yes. Cheaper alternative : grinding wheel on a cutoff tool https://www.3m.com/3M/en_US/p/d/b40065396/ On the outer panel that I will be using , I just use a regular drill bit to get through the spot weld . As already mentioned, porosity can be difficult to completely eliminate on a patch panel . I like to use Evercoat Fibertech over the weld seams. http://www.evercoat.com/images/ePIM/original/TDS_100633_FIBER-TECH-4.2016.pdf. After you are finished, try to coat the backside with epoxy primer or cavity wax with a wand to get in the tight spots.