Weld-Through Primer Coating

Weld primer is not a coating you apply before welding, but a primer that's on the sheet to protect it from corrosion until welding. The film or primer must be cleanly removed in the weld area, because the scale and organic components burn during welding and can cause pores or cracks in the weld. If you weld through the coating, you won't get a clean, load-bearing root. So: Always grind bare in the weld area, then you'll have no problems with impurities and weld quality.