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Clearing up the confusion about weld through primer

Weld through primer is one of the most misunderstood materials in auto restoration.

Weld through primers contain either zinc or copper mixed in with the coating making it electrically conductive and thus facilitating welding.

A premium aerosol to protect blind welds

While it can be used to protect bare metal when you plan to weld later, it’s true purpose is for protecting blind joints that aren’t completely sealed by the welding process. It protects the inside surfaces of the open weld and any surface not accessible to painting or sealing after the parts are welded.

This is particularly true when welding overlaps, spot welding, or plug welds where bare metal internal surfaces that are in close contact can’t be accessed (without undoing the welds) and are therefore left vulnerable to penetrating moisture.

Weld Through Primers are best used on the blind faces (in the “sandwich” of the metal) and not the outer surfaces of the joint.

The welding action doesn’t destroy the weld through primer. The metal within the Weld Through Primer liquefies (melts) and while cooling shrinks-up around the weld (or wicks-in) to seal within the joint thus providing long term protection.

Copper vs Zinc: Weld #2 is available in both

There are conflicting views regarding which is the best for different applications.

To facilitate a strong weld, steel needs to reach its melting point of roughly 1,400 to 1,540 °C. Both zinc and copper melt at below these temperatures which is a needed property so that when the joint cools the zinc or copper encapsulates the welds providing protection.

However, at these temperatures zinc is more likely to boil (turn into a gas) because it boils at roughly 910 °C. resulting in some zinc escaping the joint reducing the amount of protective metal available.

Copper boils-off at a much higher temperature (2,580 °C). It’s therefore more likely to provide superior encapsulation.

Some users also find that copper conducts electricity better, improving the welding process and producing less weld spatter.

Some believe that zinc has superior resistance to corrosion and some car manufactures recommend using zinc over copper.

For this reason, U-pol provides Weld #2 in both Zinc and Copper.

U-pol Weld Through Primer product information

U-pol WELD#2 Primer is a high adhesion, fast drying, super conductive primer offering excellent rust protection when applied to suitably prepared panels prior to SPOT, MIG and TIG welding.

Key Features of the U-Pol Weld #2 – Weld Through Primers:

  • High adhesion to metals and super conductivity
  • Superior rust protection for SPOT, MIG, and TIG welding applications
  • Dry film contains a very high concentration of either zinc or copper
  • Fast-drying formula helps enhance the welding process
  • Provides excellent reflow around the weld.

The WELD #2 COPPER has:

  • Large amounts of pure copper
  • Provides a long-lasting, semi-flexible metallic finish that adds to the corrosion protection
  • The WELD #2 ZINC has:
  • Large amounts of pure zinc
  • Provides a long-lasting, semi-flexible metallic finish that adds to the corrosion protection

Superior qualities of U-pol Weld #2 primers

Many competing Weld Through Primers require removal from surfaces to facilitate painting or sealing. Both Zinc and Copper variants of U-pol’s Weld #2 primers are formulated to be compatible with most topcoats and polyurethane sealers. You don’t need to wash-off or cut back U-pol Weld #2 for overcoating or sealing.

U-pol Weld#2 is part of U-pol’s Premium Aerosol range designed to enhance workshop productivity and deliver outstanding results.

Applying the WELD #2 COPPER or WELD #2 ZINC:

  • Clean and degrease the surface thoroughly with a good quality spirit wipe or thinner
  • Abrade the surface, wet or dry, with 280-320 abrasive, clean again, dry, and degrease
  • Shake the can well for at least 2 minutes
  • Apply 1-2 light coats, evenly, allowing 5-10 minutes between each coat
  • Provide 10-20 minutes before welding
  • The primers should be flatted wet with 600-800 grit or dry with 400-600 grit abrasive
  • Can be over-painted with most paint systems and sealers available in the market.

Weld #2 Primer Premium Aerosols are available from leading automotive paint distributors.

Find out more by calling u-pol’s expert technical custom support team on

(02) 4731 2655 or visit

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