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Finishing with Spar Varnish

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  • Finishing with Spar Varnish

    I'm using Spar Varnish on a cabinet and it appears to have bubbles in the finish when dry. I have been careful not to shake the finish to produce bubbles while in the can, and have tried using the wipe on method and again using a brush. I also cleaned my shop of dust prior to finishing (24hours) and was careful not to stir up the air too much while finishing. Any suggestions to achive that glass like finish I'm looking for?

  • #2
    Re: Finishing with Spar Varnish

    Your application method sounds good. It's a darn difficult finish to apply.

    The only suggestion I can make is to tent the project after you apply the finish. Don't let the cover touch it, but rig up a stand and cover it with something (clean) before you walk away. The best way would be to have the cover as much in place as you can, apply the finish, then pull the cover over when you're done. My thinking here is that dusties are getting to it as it dries.

    Make sure you don't put too much on at once. Go with a real light amount of finish and do multiple coats. It takes a long time, but you'll get there.

    You can sand what you've got and start again. Just be careful sanding out, and don't go deep. It'll take a hand sander, or a REALLY really careful hand with a power sander.

    I hope that helps! Good luck.
    I put it all back together better than before. There\'s lots of leftover parts.


    • #3
      Re: Finishing with Spar Varnish

      I use a natural bristle brush (Purdy or Wooster China Bristle).

      For brush on, I put the brush about 1" into the varnish, tap the edge against the side of the can, (Don't drag the varnish off the brush on the side of the can,. You want it to flow onto the surface from the brush, so you want the brush loaded as full as possible without it dripping) and then slowly apply it working from the dry surface back into the wet surface. After fully wetting a surface, i then drag the brush along the entire surface, running with the grain, with the brush almost perpendicular to the surface and just the full width of the tip riding in the varnish (called "tipping it out"). This removes almost all of the bubbles. If I find it does not, I thin the varnish about 10% and try again. It needs to be tipped out before it starts to tack up and cause the brush to drag. If the brush starts to drag, stop and let cure, then sand the ugly spot smooth before the next coat.

      For wipe on, I thin the varnish about 50%. I usually wet the surface with a brush, and then come back and wipe the surface smooth and even with the cloth pad, using long strokes along the grain. I haven't had a problem with bubbles using wipe-on. Your mixture may be too thick.

      Other things that cause bubbles are:

      Contaminants (sanding dust, etc) in the grain pores, which trap air. Solution: Vacuum surface, and then wipe with a rag dampened with mineral spirits to get up the remaining dust. I use a very lightly damp rag to wipe off after scuff-sanding between coats.

      Water moisture on the surface, causing gas bubbles;: Solution: Let it dry, and don't use compressed air to blow off the surface (If the bubbles look like craters, you have oil droplet contamination, and will have to thoroughly wipe the surface with mineral spirits or denatured alcohol, wiping the solvent on with one rag, and wiping up the wet solvent with a clean one before it dries)

      Too thick a coating, so the bubbles cannot work out the surface before the surface dries; Solution: Thin coating by 10 - 20%, or do not apply as heavy a coat. I have found many thin coats finish better than a few thick ones.

      Too hot a temp, again causing the surface to tack before the bubbles can work out of it. Solution: Wait for a cooler day, or turn down the heat.

      Hope this helps


      PS: I find that Minwax poly varnishes are bad for bubbles, but the above does work. Phenolic Resin varnishes like Behlens Rockhard Table Top Varnish, and Waterlox give me less problems, flow on smoother, level better, but cost about twice as much.
      Practicing at practical wood working