Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2008

Pinholes (Gasing)

Q. WE ARE PAINTING OVER GALVANIZING WITH EPOXY PRIMER & POLYURTHANE FINISH. WE KEEP HAVING PROBLEMS WITH PINHOLES, IT SEEMS LIKE THE GALVANIZING IS STILL GASING WHICH CREATES A BUBBLE. WE'VE BEEN GIVING IT MIST COATS BUT IT DOESN'T SEEM TO BE WORKING. WHAT CAN WE DO?

A. One reason for experiencing pinholes from galvanizing is that there are small air pockets on the galvanized surface that escape once the primer is aplied. If the primer and top coat forms a skin too quickly, the viscosity of the paint is too high to flow over and close the pinholes as the air leaves the coating. You might try to slow down the drying process by using a slower evaporating solvent. Alternatively, ask your paint supplier if he can use a different flow control agent in the primer that allows the coating surface to remain wet for a little longer.

If you place the primed parts into an oven, I suggest you wait longer before subjecting the part to heat.

To prevent or minimize pinholes your goal should be to allow the air to escape before the paint viscosity increases to the point where is can no longer flow back to cover the holes.

Regards,

Ron Joseph