Stubborn bubbles in poly

[Mike Hoyt]

Hey there everyone, I just encountered an odd issue while finishing some red oak with gloss oil based poly. It went down fine, leveled OK and seemed to be curing well, it was after the second coat was mostly dry that I had began to have a few small bubbles start to grow. Almost as if there was some type of pressure behind them. I'm really stumped because I have never run into an issue like this and as I said, the second coat was nearly dry so the bubbles of course didnt pop. Any ideas?? maybe thin it a bit ?

Thanks,
Mike

[Neal Clayton]

what's under the poly?

[Mike Hoyt]

Neal, all i did was sand to 120 and apply the clear poly over the bare wood. It was only about 75 degrees here that day, my only thought was maybe some of the larger pores held some finish and for some reason it surfaced as the poly was nearly dry. I'm kinda stumped on this one
Mike

[Ken Fitzgerald]

Mike,

Was the temperature rising when you applied the poly or had you stored the wood in cooler place and brought it to a warmer or warming place?

Hoadley IIRC in his book Understanding Wood remarks he had problems when in the winter he brought some wood upstairs from his basement to the warmed section of his house and applied finish to it. He said that he thought that the warming of the wood caused the wood to expand and it released trapped air bubbles into the drying finish.

[Neal Clayton]

i've seen similar to that ken, a couple of years ago on a heat stripped piece that i tried to put poly on. the heat of course causes some movement and gets any remaining sap moving. lots of bubbles.

since then i've stopped using poly altogether but i think that's a legitimate hypothesis.

[Howard Acheson]

As an oil based finish dries and cures it off-gases. With large pored wood like red oak, the deep pores get filled with finish. The top of the finish skins over initially but the deeper down finish dries much more slowly. If you put on a second coat too soon, the off-gasing finish in the deep pores will cause bubbles in the wet top coat. It's sort of a bleed back problem.

Red oak can present some finishing problems. I'd let it fully dry for a week or so and then sand it flat with 320 paper. Then apply another coat of finish.

[Mike Hoyt]

Thanks guys for the ideas, and IIRC it was warming up as the day went along so that certainly may have played a part. Today I applied the second coat to the other side, this time I didnt have any bubbles at all and the only thing I did differently was thin the poly just a bit. It was much cooler today and obviously the finish cured much slower so I'm not sure which of those variables made the change in results but at any rate I think I'm all set. I will let that coat dry a full 24 hrs before sanding and proceeding so wish me luck, thanks again everyone. This experience just adds to my dislikes of red oak.

Mike

[Rod Sheridan]

Thanks guys for the ideas, and IIRC it was warming up as the day went along so that certainly may have played a part. Today I applied the second coat to the other side, this time I didnt have any bubbles at all and the only thing I did differently was thin the poly just a bit. It was much cooler today and obviously the finish cured much slower so I'm not sure which of those variables made the change in results but at any rate I think I'm all set. I will let that coat dry a full 24 hrs before sanding and proceeding so wish me luck, thanks again everyone. This experience just adds to my dislikes of red oak.

Mike

Mike, your dislike of red oak is understandable, however in my opinion unfounded.

I'd be more critical of the poly.

I've found that it's too thick to do a good job, and I thin it from between 20 to 50% before application, depending upon what I use it on.

You may have solved your problem by thinning it.

Regards, Rod.

[Prashun Patel]

I've had shellac do this to me. Jeff Hewitt explained that this can happen when a coat is not fully cured and all the solvent is not completely evaporated. The solvent eventually makes its way through the skinned second coat, causing bubbles. Heat will cause the top coat to harden quicker, which will cause more trapping and more bubbles.

You can solve this next time by thinning your coats or waiting longer between coats, and not exposing the piece to heat or sun while curing.

For the CURRENT piece, the good thing is that you haven't colored the wood. If it were me, I'd wait a few days, then sand back to level out the bubbles as best as you can (wetsand with 320 or even 220gt) without creating witness lines. If yr experienced with a card scraper, that is even quicker and cleaner. Then put another thinned coat of poly on top and see if the prob is solved.

Worst case, I'd scrape the finish off and restart. Scraping varnish off is IMHO way easier and better for the surface than trying to sand it off.

[Mike Hoyt]

Just wanted to update everyone on the outcome. I layed down 3 coats on the other side of the treads this time thinned 20% and the results were immediately better. The finish leveled out much faster and smoother and no bubbles at all. Thanks again for all the help!!
Mike