Polyurethane Finish Help

[Paul McCormick]

Hi –

I built a table a few years ago. The top is ¾-inch, cabinet grade plywood with a wood banded edge. I used Pore-O-Pac filler, stained with Minwax gel stain, and put 4 thin coats of Helmsman Spar Urethane over that for the finish. I sanded lightly between coats, and used a buffing pad on the final coat to polish it up. Looked great when I finished.

Over the years the finish has started to bubble and peel up in big sections. The poly peels off in sheets, bubbling up in areas the size of a dime to as big as 6-inch circles.

I let the pore filler dry for a week, the stain for 24 hours, and each coat of poly for at least 24 hours. The workshop I built the table in was unheated, and it was in the low 60s in there when I was working. The only thing I can think of (other than I just totally screwed it up) is that the finish didn’t completely finish drying due to the cooler temp, and the finish has continued to off-gas, causing the bubbles.

I’m getting ready to scrape and sand the whole top and redo it, and I want to avoid a repeat of the finish delaminating. If anyone has any advice or ideas, please share.

Thanks,
Paul

[Jim Becker]

Any finish that has polyurethane as part of it's nature "doesn't like to stick even to itself"...said in a chuckling way, but honestly, that's not far from the truth. Sanding between your coats was a good idea for adhesion, but it sounds like your top coat finish wasn't compatible with what came before it. You'll find that many of us use de-waxed shellac as a barrier coat between the oils/stains/dyes/fillers, etc., and our top coats to help insure that's not an issue. The fact that your finish is peeling off "in sheets" is indicative that there was limited adhesion in the first place. You really will likely want to strip the piece (chemically), sand lightly and then start over.

But let's also step back a second here. Spar varnish, a long-oil varnish, isn't a particularly good choice for durability, especially for interior projects. Spar varnish was designed to be flexible since spars (components on boats) live in the weather and move a lot from temp and humidity and it's also very soft. So for your re-do, use a finish that's designed for interior use, either an oil based top coat varnish if that's what you're most comfortable with because you're brushing or wiping it on, or a water borne top coat finish via spraying. You don't need to use something with polyurethane in it, either. Many of us don't for a variety of reasons. Sherwin Williams, Pratt and Lambert and others have great oil based varnish products that are "poly-free". Waterlox is a popular oil based finish with many folks who want an oil based finish. All of these can be brushed on or wiped on (properly thinned if required) so they are easy to use. Do note that if you choose to wipe-on your finish, you'll want to do a minimum of 12 coats to get somewhat the same build as with three coats brushed. And consider a barrier coat between the "pre-stuff" and the top coats, too.

Oh, and get the temp up while your finishing and it's curing! The low 60ºF-s is generally below the required range for these products to cure properly. Remember, they cure chemically and that chemical reaction is affected by temperature.

[Scott Holmes]

To add to Jim's advice... The low temps will SLOW the curing process, they will not stop it or harm it. As soon as I read your post; I thought: "Spar urethane is too soft and probably didn't stick well to the Pore-O-Pac filler.

Waterlox is an excellent varnish one of my favorites... be fore-warned it is a slow drying varnish 8 hours plus between coats... Worth the wait... after 30 days or curing you can buff it up to an incredible gloss shine.

Another warning: A high gloss finish will highlight every flaw in the finish; a satin finish will hide most flaws in the finish.

[Stan Calow]

T
A high gloss finish will highlight every flaw in the finish; a satin finish will hide most flaws in the finish.

Scott that is an excellent piece of advice. I don't think I've heard that as clear and simply put before.