Polyurethane question - Woodworking Talk - Woodworkers Forum
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post #1 of 7 Old 03-27-2019, 04:17 PM Thread Starter
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Polyurethane question

First time I'm seeing this.

I'm refinishing an old 2x pine dining room table, I sanded the top clean up to 320 grit, (no stripper), applied stain, then a seal coat of dewaxed shellac. When I applied the poly I'm seeing a few spots where it seems like the poly is moving away, not big areas (about 1/8" dia.) just here and there. This is a big table with leaves I don't want to start from scratch on it, any ideas, also the shellac is new.
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post #2 of 7 Old 03-27-2019, 05:03 PM
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Originally Posted by redeared View Post
First time I'm seeing this.

I'm refinishing an old 2x pine dining room table, I sanded the top clean up to 320 grit, (no stripper), applied stain, then a seal coat of dewaxed shellac. When I applied the poly I'm seeing a few spots where it seems like the poly is moving away, not big areas (about 1/8" dia.) just here and there. This is a big table with leaves I don't want to start from scratch on it, any ideas, also the shellac is new.
If what is on the left of this picture is the problem you are having it's called fisheye. It's common to get it in furniture refinished. It's caused by silicone most commonly found in aerosol furniture polish. It can go all the way through a finish and get into the wood. Even if you had chemically stripped the piece it wouldn't clean off the silicone. You would have had to wash the furniture off with a wax and grease remover prior to doing anything.

The only fix for fisheye is to add silicone to the polyurethane. A place that sells automotive finishes will have some type of fisheye control solvent to add to the finish. I use one called smoothie. Now, keep in mind the fisheye control solvent is silicone and you need to be clean with it. Any rags, sandpaper or anything you use with the table you need to either dispose of or put them in a marked container when you are done. If not you could build a project with new wood and sand it with the same sandpaper used on the table and when you go to finish it the finish will fisheye. If you are spraying the finish you need to be careful where the overspray goes as well.

You got lucky refinishing your table by sanding. Sanding tends to get what is on the surface and leaves finish in spots penetrated in the wood. Then when you use stain you find places that won't accept the stain. Any refinish should begin with a paint and varnish remover.
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post #3 of 7 Old 03-27-2019, 08:30 PM
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I had this happen on a coffee table I pulled from the neighbor's trash. I refinished it for my Daughter's new apartment (flown the coupe). The top was veneered very nicely so all it got was a light sanding. I laid on the best application on the top I had ever put down. and watched in horror as the poly began to literally bead up. My first lesson in silicone contamination. The neighbor told me the table had been his Mother's and she really laid on the Pledge. I wiped the poly off with tons of mineral spirits before it could set up. I then covered the silicone contaminated top with dewaxed shellac and then did the poly finish with no issues. The problem with silicones is that you can't get rid of them, only cover them up, or follow what Steve said about adding fish eye preventer to the finishing material. However, once you introduce silicones into your finishing stuff, you are stuck with them forever or your toss everything you used that had the fisheye preventer in the finishing materials. I've used Smoothie with automotive lacquers with good results, but I no longer have the spray gun used for that function.

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post #4 of 7 Old 03-27-2019, 08:55 PM
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I had this happen on a coffee table I pulled from the neighbor's trash. I refinished it for my Daughter's new apartment (flown the coupe). The top was veneered very nicely so all it got was a light sanding. I laid on the best application on the top I had ever put down. and watched in horror as the poly began to literally bead up. My first lesson in silicone contamination. The neighbor told me the table had been his Mother's and she really laid on the Pledge. I wiped the poly off with tons of mineral spirits before it could set up. I then covered the silicone contaminated top with dewaxed shellac and then did the poly finish with no issues. The problem with silicones is that you can't get rid of them, only cover them up, or follow what Steve said about adding fish eye preventer to the finishing material. However, once you introduce silicones into your finishing stuff, you are stuck with them forever or your toss everything you used that had the fisheye preventer in the finishing materials. I've used Smoothie with automotive lacquers with good results, but I no longer have the spray gun used for that function.
Back when I had a refinishing shop I used to spray the back side of sandpaper and sanding pads with a bright red paint to indicate it was contaminated with silicone. I didn't seem to have any trouble with the sprayer getting contaminated. The silicone seem to easily rinse out with lacquer thinner. I just had to make sure I didn't spray a clean piece of furniture with finish mixed with smoothie.
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post #5 of 7 Old 03-27-2019, 08:59 PM Thread Starter
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Thanks I do remember now about reading about that some time ago. I guess the seal coat of shellac covered most of it or it could have been worse.
Now that it is dried it is not as noticeable, and the people want it to be buffed down to a matte finish so I think I'm good.
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post #6 of 7 Old 03-27-2019, 10:53 PM
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Thanks I do remember now about reading about that some time ago. I guess the seal coat of shellac covered most of it or it could have been worse.
Now that it is dried it is not as noticeable, and the people want it to be buffed down to a matte finish so I think I'm good.
The shellac didn't cover up the silicone, the piece you worked on just didn't have enough silicone in it everywhere to cause fisheye. Silicone will mix into any finish you put on the wood and float to the surface. You could put coat after coat of shellac on and the silicone would just keep coming to the surface. It's why a lot of furniture refinishers curse aerosol furniture polish.
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post #7 of 7 Old 03-30-2019, 10:00 PM Thread Starter
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Well I stripped the whole table, then used paint remover with steel wool and refinished it again, I ended up with maybe four pin head spots which I can live with.
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