Bubbles are usually caused by not sealing wood or canvas surfaces effectively. All wood and canvas surfaces require one brushed-on layer of epoxy coating to seal the wood. Use one-ounce per. sq. ft. for "seal coat". Oak, cedar, cypress, redwood, barn wood, hickory require two layers of seal coat. Three layers of Poly or Varnish may be substituted for seal coat on other woods. Poly must be sanded before the coating is applied.