Epoxy Adhesion Over Stains

by Tom Pawlak — GBI Technical Advisor

Above: Testing of epoxy adhesion over stains was done with a number of different stains.

We have occasionally reported results of adhesion testing of WEST SYSTEM® epoxy to wood coated with various stains. In “Varnish Over Epoxy,” we touched on the topic of wood stains under epoxy and warned that some oil-based stains would cause an adhesion problem for WEST SYSTEM Epoxy. Since then, we have done some additional adhesion testing of epoxy over a number of stains.
These included paste wood filler stain, oil-based stains, and water-based stains. This testing also compared the adhesion of epoxy to stains that had dried one day to stains that had dried four days. We tested epoxy adhesion over stains using our variation of ASTM-3359-83, which involves scoring the epoxy coating with a razor blade. Two to three 2″ long cuts are made parallel to one another about ¼” apart, deep enough to pass through the epoxy. Another set of cuts is made to intersect the first at a 45° to 90° angle. The points of intersection create ideal places where the epoxy coating will peel off if adhesion is poor. In the ASTM test, a special 3M™ tape with high tack is applied over the scored area and pressed into the coating to assure thorough contact. We substituted quality duct tape with high tack strength for the 3M tape.

Epoxy will peel from the stained surface and remain attached to the tape.

Once applied, the tape is pulled back at a flat angle, quickly, with a firm tug. If adhesion is poor, the epoxy will peel from the surface and remain attached to the tape (above). If adhesion is good, the epoxy remains attached to the stained surface.

Epoxy Adhesion Over Stains — Test Results

Stain brand/type 24 hr. dry 4-day dry
Pettit™ Paste Wood Filler Stain fail (see photo) pass
Minwax™ Wood Finish (oil base) fail fail
Minwax Gel Stain pass pass
WD Lockwood™ water-base aniline dye stain pass pass
Minwax water-base stain pass pass
Pratt & Lambert™ Tonetic oil-base stain pass pass
Old Masters™ oil-base (wiping) stain pass pass
Ace Brand™ oil-base stain pass pass
Bare wood control pass pass

These results show that WEST SYSTEM Epoxy adheres to some oil-base stains after 24 hours of drying. Others require more drying time before epoxy will adhere reliably. Our epoxy did not stick to Minwax Wood Finish even after four days of drying under ideal drying conditions.
If you have a preferred stain not listed above, you can conduct your own adhesion test. Be sure to cut completely through the epoxy coating and use a high-tack tape. If you have no preferred stain, consider WD Lockwood’s water-based aniline dye stain. It has worked reliably under epoxy for many years.