Epoxy, Urethane Bubbles, And Application

[FC]

The following information is very helpful to those who have bubbling when they apply polyurethane finishes.

 

I use System Three Epoxy, blacked with Brownell’s Epoxy Black to fill in gaps in my mesquite. It makes a great base for urethane finishes.

 

From System Three Resin’s Epoxy’s booklet, 1992

 

“Try to work at a constant or falling temperature when coating new wood. When the temp. is rising, air trapped beneath the uncured epoxy may expand and cause small bubbles to form in the coating. Avoid working in direct sunlight on new wood for this reason. If you must work in sunlight, coat the wood as the sun is going down. The wood will be cooling and air bubbles should not form. Evening dew condensation does not harm the partially cured System Three epoxy coating.

 

Some very porous woods are quite persistent at forming air bubbles. A trick we have used is to heat the whole surface to a temp about 40 degrees F. or more higher than room temperature. Use a hot air gun, or place the wood in the sunlight for awhile. Stop heating and immediately coat the surface. The epoxy (or finish) will thin on the warm surface and at the same time start to cool it. The air in the small pores will begin to contract, pulling the thinned epoxy (or finish) into them. Any air that does rise will be going through thinned epoxy and have an easier time of it.”

 

Epoxy- do not overclamp. The higher the temperature you are working, the faster the set. Curing occurs over several days. Joints come under stress when unclamped and need additional cure time when clamps are removed. Overnight set is usually enough time for non-stressed joints.

 

Wear disposable gloves- you can get burned badly when the exothermic reaction gets going! Use a tongue depressor or thin piece of, say ½ to 1 inch wood to move the epoxy around and shove it into the joint or gap. Keep joint level! I lifted a piece after an initial set and ended up with epoxy puddle on my garage floor. It sands well, but use a dust mask. Use silica epoxy thickener if filling gaps if it isn’t a clear finish.

 

I get “pucker” or “cratering”and have to go back over the gap as the epoxy dries and scrape the epoxy in there to build up the site. I sometimes refill the joint two or three times to get the surface level. It’s kind of a shrink factor. Epoxy does go through the planer well.

 

“Epoxy coatings should be sanded before varnishing or painting. These materials stick to the epoxy by mechanical means, and must have some “tooth” in order to bond well.

 

“All outside epoxy surfaces exposed to sunlight must be must be protected from degradation by UV light.” The effect over time is dulling of the clear film, chalking, then cracking and deterioration. “Opaque paints do not allow the passage of UV…” Primer is beneficial. “If it says alkyd anywhere on the can then it is an alkyd, not a polyurethane.” “LPU paints dry very hard with excellent gloss, are not degraded by sunlight, and wear very well. Their main drawback is that they require immaculate surface preparation. The solvent based LPU paints contain some very toxic materials and are difficult and touchy to apply. They can turn dull when curing if the humidity is too high.” This stuff is difficult to apply correctly. Since we tend to use varnishes, urethane, or shellac, I’d go with a marine varnish and try to keep the gun from standing in the sun all the time.

 

[ken98k]

Thanks!

I didn't know epoxy was/is affected by sunlight.