Am I allowed to weld over paint? Or do I have to remove it? What about primers and paints that claim to be “weldable”?

Question:Am I allowed to weld over paint?  Or do I have to remove it?  What about primers and paints that claim to be “weldable”?

Author: Karsten Madsen: President – OptiWELD: Optimized Welding Services 

Answer: The breathing zone of welding personnel may contain fumes from a combination of base and filler metals as well as surface coatings including paint in some cases.  All of these ingredients mix in the heat of the weld puddle with air-borne particulate forming the fume that can be seen as a plume rising from the weld area.

Considering that at least some paints may contain toxic ingredients, it may be best to remove all paint from at least 10 cm away from all joints prior to welding.  There may also be a requirement to remove paint for code compliance.  For example, clause 9.4.1 of CSA W59 states "Surfaces of old material that are to receive welds shall be cleaned thoroughly of all foreign matter, including paint film in the area immediately adjacent to the weld".  Keep in mind that if paint is removed by grinding, you still run the risk of airborne contaminants.

Some paints are considered "weldable" although that terminology may relate more to potential influence on weld quality vs personnel safety.  Weldable primer is often used to prevent corrosion during storage of metals prior to final processing.  These primers are usually zinc-based products that can lead to "metal fume fever" with flu-like symptoms from over exposure.  Although there are no long-term effects from this exposure, it may be best to wear a suitable respirator if welding over these materials.  

More concerning would be welding over painted metals containing other materials such as lead-based pigments.  Lead of course is a very serious toxin with cumulative effects so even small exposures must be avoided.  If parts must be welded that have been painted with lead-based pigments, suitable control methods should be used including air-supplied respirators with exposure monitored by blood testing of personnel.

In all cases when asked to weld over painted parts, you should review the Material Safety Data Sheet (MSDS) of the specific paint used.  The MSDS will list the proportion of potentially hazardous materials, their exposure limits and suitable control methods to use.  This analysis should provide information on potential hazards so paint removal or suitable exposure control methods can be used to weld safely.  For due diligence, this can be followed up with air quality monitoring to verify potential exposure to breathing zone hazards going forward.