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What does I-CAR say about how to use weld-through primer?

Response

Per I-CAR curriculum: "Joining surfaces can be the source of corrosion hot spots if not adequately treated.

After straightening and cleaning weld mating surfaces, a weld-through primer is often applied for corrosion prevention. Weld-through primer:

  • is designed to be used for both GMA (MIG) weld and spot weld mating surfaces. This includes butt joint inserts, when the insert is not E-coated.
  • is heat-resistant, so it will not burn away during the welding process as much as other coatings would.
  • may be zinc or copper based. Claims of 95% zinc or copper content mean that 95% of the coating left after the primer has cured is zinc or copper.
  • is available as a spray or brush-on.
  • is not a good base for topcoats. Any excess weld-through primer should be cleaned off before applying other coatings.
  • may need to be dry to the touch before welding. Some can be welded when the coating is still tacky. For example, 3M weld-through primer needs to be dry before welding. The Sherwin-Williams Shield “thin-film technology” product can be welded while still tacky.
  • is applied to clean, bare metal surfaces that cannot be protected by primer or anti-corrosion compound. These areas include flanges and inserts or reinforcements that are flush with the part.
  • should not be applied too thick. One thin coat of brush-on or two light coats of aerosol are often recommended. Applying too much will increase resistance to the welding process resulting in excess heat and weld spatter.
  • should be removed from the weld zone before GMA (MIG) welding. Welding through weld-through primer may cause porosity in the weld. This requirement may not be clearly stated on the product container or even the product data sheet.
  • varies by the amount that burns away when a GMA (MIG) weld is made."