polyurethane Problems

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Old 06-03-11, 04:45 PM
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polyurethane Problems

I've had this problem before but never got a good answer for it. I am refinishing a table. I've sanded it down, cleaned it off, applied 4 layers of Minwax stain over the past week. Let the last coat sit for about 7 hours then applied a layer of polyurethane. After the first coat of polyurethane, when it started to dry, I noticed white streaks on certain areas of the wood. It almost looks like someone took a piece of white sandpaper and tried to scratch it. It looks like its under the polyurethane but it wasn't there to begin with. All the materials are oil-based. It is humid here and I am, unfortunetely, having to do this project outside in the garage. PLEASE help.
 
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Old 06-03-11, 08:49 PM
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Was the can of polyurethane shaken before you used it? It's never a good idea to shake polyurethane because it introduces air bubbles into it, and those can be a nuisance. Everything short of high gloss poly should be stirred before use. Still, one would expect those bubbles would be large, not extremely tiny.

I'd try taking a hair dryer to those white discolourations and see if they disappear.

Whenever you see anything that's white in colour and that shouldn't be there, the door flies wide open on what could have caused that white discolouration.

Refraction of incident light at any interface where the refractive index changes cause that light to scatter into a spectrum of different frequencies of light that all go in a slightly different direction. Subsequent reflections and refractions cause the incident light to be scattered so that your eye is seeing different colours of light from different places all at the same time. Your eye sees that scattered light as the colour "white". This is precisely why clouds, snow banks, waterfalls and the head on a beer are all white, even though nothing inside any of these things is actually white in colour.

Consequently, anything that can cause the scattering of light will create a white discolouration. On lacquered furniture, it's common to see "water marks" where someone has left a cold beer. The condensation has dripped down the bottle and individual molecules of H2O on the surface of the lacquer have migrated into the lacquer and accumulated in extremely tiny voids in the lacquer in large enough quantities to form liquid water. It's these astonishingly small droplets of water that form a cloud right inside the otherwise transparent solid lacquer coating. By heating the lacquer with a hair dryer, you can cause the individual water molecules to evaporate from the droplets, and thereby eliminate the solid/liquid phase boundaries inside the lacquer where the refraction of light is occuring. And, by doing that, you eliminate the white discolouration, which is the water mark.

I'm thinking the same thing may have happened in your polyurethane due to the moisture content of the wood.

If heat doesn't do any good, then I'd be most inclined to use an abrasive pad (like a green Scotchbrite pad of the kind sold in grocery stores for scouring pots), damp with glycerine (which you can buy at any pharmacy) and try to scrub the white discolouration out of the polyurethane.

Glycerine behaves very much like a light lubricant, but is an alcohol. As a result, the glycerine you use will evaporate from the wood completely without leaving a residue. And, I'd wait a full week to allow the glycerine absorbed into the wood to fully evapoate before applying another coat of poly.

Any residual glycerine SHOULD evaporate through the fresh polyurethane film. When they tint polyurethane paint, the colourants they add to the paint use glycerine as a carrier fluid.
 
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Old 06-04-11, 03:44 AM
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Back when I was an apprentice the old timers would always say you can't apply varnish in damp weather because it will dry cloudy. Personally I've never had that problem so I believe it was an issue with shellac not varnish or poly.

It's never a good idea to apply multiple coats of stain. Stain is formulated to be absorbed more so than to lay on top of the wood. Multiple coats typically don't dry as well and can be rewetted by the solvents in the poly/varnish.

I'd let it dry another day and see how it looks. If it's still there, sand lightly and then wipe it down with a thinner rag to see if it's still there. If sanding removes it, a slightly thinner coat of poly should finish the piece. As noted above - never shake poly, always use a stir stick.
 
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Old 06-04-11, 04:55 AM
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Thanks for the replies.
I made sure not to shake the can. I took a clean screw driver and stirred it. The piece this happened to was a chair. I wanted to poly something that was small so that I could see if it happened to me again, as said earlier, its happened before. Anyway, I wasn't even able to get all the way done polyurethaning the chair before the "white" streaks appeared in a part of it. I went to try and wipe it off but it was already dried before I could. The poly is a fast dry polyurethane. Now, the reason I say this is to ask whether I should use a blow drier while the polyurethane is dry or immediately after I put it on with the paint brush. Like I say, I have almost no time before it dries
A little background may help as well. This is the second table I've restored and its happened to the first one as well which was polyurethaned inside my house. However, I restored a wood floor, putting on multi-coats of stain and polyurethane without any incident. Hope this helps alittle more and thank you again for the advise.
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Last edited by mystang89; 06-04-11 at 06:13 AM.
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Old 06-04-11, 09:11 AM
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Is the wood rough where the white is? How are you applying the poly?

The wood doesn't appear to have multiple coats of stain

Adding a little thinner to the poly should give you a little more 'work' time. I doubt using a hair dryer would help anything.
 
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Old 06-04-11, 09:59 AM
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I tried 4 chairs using different methods. One chair was using the brush I had already used, the next was using a foam brush, the 3rd was using a Purdy 4" brush and the last was using the spray on polyurethane. Each chair came up with white marks in different spots but all of them, except for the spray on which hasn't dried yet, have white marks on the corner. The wood is all smooth and it has 3 different coats of "Cherry" stain on it which is probably why it doesn't look like there is a coat on there at all.....talk to the wife not me, I wanted it darker. Anyway, that is where I'm at now. I can't tell if maybe I'm not putting it on thick enough and, since its out in the sun that its drying too quick so maybe I'm smearing it once it does dry. Though, it seems if that was the case that it would look like that on all the chairs. I don't know. I'm completely stumped.
 
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Old 06-04-11, 12:45 PM
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I'd try a different polyurethane.

I've never seen anything that dramatic on anything I've varnished.

Try using Minwax's Wipe-On Polyurethane. It's much thinner than brush on polyurethanes since it's meant to be wiped on with a rag.
 
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Old 06-04-11, 03:58 PM
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Generally it's best not to apply paint in the sun [except during the winter ] Moving the items to be poly'd or maybe pulling the garage door partly down so the items are in the shade should help to cool the piece off some and give you a little longer work time. Thinning the poly a little will also give you a little more leeway in the application.

IMO a wipe on poly won't give enough protection - the mil thickness of the poly just won't be there..... but while my opinion is formed from years of painting, I've never actually used any wipe on poly.
 
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Old 06-04-11, 06:00 PM
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Well, while I haven't had the opportunity yet to try and polyurethane the pieces in the garage itself as the day escaped me, I did find a way to remove the white streaks while at the same time not completely removing the polyurethane. I took a scotch pad and put a bit of Mineral Spirits on it and scrubbed on the white streaks. After a bit of time the white streaks diminished and the polyurethane, or at least most of it, was still there. Tomorrow I will try to apply a coat on a chair I haven't already polyurethaned in the garage and see what happens.
 
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Old 06-27-11, 04:17 PM
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I would definitely try a different polyurethane paint, I haven't seen something that drastic before
 
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Old 06-27-11, 04:18 PM
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Make sure you don't apply it in the sun it can mess it up, that problem seems weird
 
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Old 07-15-12, 07:18 AM
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I have exactly the same issue. I'm using minwax semi-gloss poly.
I'm doing a set of wooden doors and its extremely frustrating as having done all the work with sanding and multiple coats of stain. This is the final step. As I have been having the problem with each of my doors, I've managed to eliminate sanding, contamination, lack of stirring and humidity (used a dehumidifier) as the issue. I am at my wits end so I am moving on to a different product. I would like to know if anyone has any ideas on how to remove the white streaks without effecting the stain. Is there something that will penetrate and/or remove the poly without effecting the stain? I tried light use of formby's furniture refinisher (the new stuff) but it made a mess. I'm trying the blow dryer method and will post the results.
 
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Old 07-15-12, 02:32 PM
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The glycerine worked to remove the white streaks! Thanks Nestor! May need to repeat application. Seemed to lighten the stain slightly so will apply another coat.
 
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Old 07-16-12, 03:50 AM
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Welcome to the forums Erik!

It's rarely a good idea to apply more than one coat of stain. Interior wood stains depend on absorption to help it dry. The first coat of stain somewhat seals the wood preventing anymore stain from being absorbed properly. A better fix would be to apply a coat of sanding sealer or poly and then tint the next coat to achieve the desired color.
 
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Old 07-16-12, 05:59 AM
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Multiple coats of stain is a bad idea - if one coat does not get the color you want, you picked the wrong color in the first place.
 
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Old 10-28-13, 09:36 PM
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I am a log home restoration contractor and I've delt with interior acrylics and other coatings fogging up/whitening. I've always contributed the problem to moisture and UV coming in through windows. I will use the heat gun (on low) and the Glycerine this weekend.
 
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Old 09-10-14, 08:41 PM
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Air buble and hole on polyurethane slab

i have a problem here..the hole have been produced moreover at the surface of the slab in production. anyone can suggest me how to overcome this problem?
 
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Old 09-11-14, 05:19 AM
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Is the picture of the top or bottom?

If the picture is the bottom (you poured the slab, then flipped it over and took the picture) then I think you have air pockets.

Here is what I do when Urethane casting. Make sure the mold is ready. Then mix the resin being very careful to not introduce air bubbles. Once the resin is mixed I put it into a vacuum chamber for 5-10 minutes depending on how much work time you have with your material. I gently vibrate the chamber (tap on the sides, use a palm sander...) to help bubbles rise to the surface. If it is a fast setting compound I refrigerate the components and mix them cold. The vacuum makes any trapped air expand into bigger bubbles that can rise and get out of the liquid.

Then when pouring gently pour some resin into the mold. Smear it in a thin layer over every surface to insure resin is touching the mold surface everywhere. Pay special attention to corners or other tight places. Then gently pour in the resin in one spot and push it gently into the rest of the mold. Moving around when pouring can trap air. After the pour vibrate the mold. Tapping with a hammer or a vibrating palm sander works well. You can also pick up (less than 1/2" or 12mm) and gently drop the mold to shake bubbles loose.
 
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