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I'm having trouble with polyurethane (Helmsman semi-gloss) drying much glossier than it should - like an extra-gloss instead of a semi-gloss. I'm using it on stained wood moulding, and when I did the majority of it in the winter, I got the expected semi-gloss sheen. But now It's coming out extremely glossy and does not match the rest (which is already installed). It's the exact same product (although a new can) and I even tried buying another can in case the can was defective, but I'm getting the same result. The only thing that seems to be different is the weather and possibly the conditions (heat) in which the cans were stored. Can exposure to heat in the can or while drying affect the sheen of polyurethane? Is there anything I can do to compensate?

Some pics:

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  • Did both new cans have the same batch # ? – mikes Jul 4 '12 at 15:06
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    Nope. And the first is 1gal; the second is 32oz. – R.. Jul 4 '12 at 15:10
  • Adding a couple quick cellphone pics.. – R.. Jul 4 '12 at 15:13
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+50

The gloss in clear finishes is reduced by flatting agents which are suspended in the clear finish. Flatting agents are tiny particles that reflect some of the light back, thereby reducing the sheen of the finish. If they settle out or are not mixed well, then the finish will be more glossy, so it's possible you did not mix the finish as well the second time.

Finishers often start with full gloss for the first coats, and then finish with the lower-gloss levels desired, so you may be able to fix the problem simply by applying properly mixed finish over the existing finish.

Another way to reduce the gloss level is to essentially scuffing the surface with fine scratches. This is sometimes called 'rubbing out' the finish. You can do it using various powdered abrasives, fine sandpaper, or scotch-brite pads and sanding with the grain of the wood. The advantage to the mechanical scuffing is that it gives you complete control over the degree you reduce the gloss. However this treatment can be much more time consuming and this would be more typical for fine furniture rather than for trim.

You can read more on adjusting sheen of a finish in this article written by Bob Flexner, a very well known wood finishing expert.

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    So far I'm a bit more inclined to believe this answer since flattening agents settling seems a lot more plausible when the polyurethane is at very low viscosity from the heat. The idea of the gloss settling (in the other answer I commented on) doesn't seem to make sense; my brush is pulling from the top of the can, not the bottom... – R.. Jul 12 '12 at 4:29
  • edited to correct my spelling of 'flatting' agents and to add a reference link to more info – JayL Jul 12 '12 at 5:12
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    I wish I had more time to evaluate the answers before the bounty expires, but since I'm going to be away the next few days, I'm awarding it to this one. The link is informative, and it seems the most plausible. I installed the moulding a few days ago (before getting responses) after having sanded and re-applied the polyurethane (mixed somewhat better but perhaps not well enough), and it's not perfect, but better. I might try going back and "rubbing out" the finish later if the gloss continues to bother me. – R.. Jul 14 '12 at 8:29
  • I agree remember to stir poly don't shake it or it will be full of bubbles. I do shake it but have a special lid and small vacuum pump to pull the air bubbles out takes a few minutes and not worth it for most DIY but that way my finish is consistent.+ – Ed Beal Feb 18 at 18:52
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Can exposure to heat in the can or while drying affect the sheen of polyurethane?

No.

Is there anything I can do to compensate?

Yes...

As a hardwood flooring installer for 4 years, I often recommended against any kind of gloss finish (It looks great at first, then it looks horrible for awhile, then it looks okay - but isn't glossy anymore). I often recommended a satin finish, which would avoid your problem... The reason you are likely getting different results are:

  • Different finish batch numbers
  • Inconsistent can mixing
  • Delayed application times

The 'glossy' look is actually an additive to the finish, and it settles in the can, and it can settle in your pail while trying to apply the finish if you are not right on top of keeping it mixed. Options to correct your issue:

  1. You can go to your local hardware store, get a 180-220 grit sanding screen.
  2. Lightly buff and sand your trim.
  3. Apply another coat of finish, making sure your can is large enough to do the whole project (or cans have same batch number).
  4. Make sure to get the finish well mixed, and keep it mixed as you go...
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    One answer says the gloss is an additive that settles, another says flattening agents are an additive that settles... – R.. Jul 12 '12 at 4:27
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    I've actually seen it as both depending on the brand and the formula. Regardless of which brand, the end result is the same. Un-properly mixed, all all of the gloss is either at the top or the bottom which results in your unwanted results. – ShoeMaker Jul 12 '12 at 11:43
  • +1 for "make sure to get the finish well-mixed". If you used up the lighter half of the finish last time, and it was matte, now you might be left with the super-glossy rest of the ingredients. – Alex Feinman Jul 12 '12 at 14:59
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    I would love to see a reference to some product that has the gloss as the additive, because I don't think it exists! I suspect the people believing this are simply misinterpreting instructions to 'stir completely to ensure a consistent gloss level' or something to that effect. – JayL Jul 12 '12 at 15:26
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    @Alex: In general that's good advice, but since my question specifically mentions new cans, it's not the issue at hand here. ShoeMaker: The question tells the exact product I'm working with. Back to the answer itself, I think the first "no" contradicts the rest of the answer. Since viscosity is greatly reduced with heat (the product was like syrup in winter and like milk in summer), any settling effects should presumably be increased with heat... – R.. Jul 14 '12 at 8:34
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I have had the same result from improper mixing and weather combination. I have been able to "knock it down" by first applying lemon oil to the piece and then using a fine sanding sponge pad or fine scotchbrite pad and working all of the way down the piece in 1 - 2 foot increments. You don't need to "grind" the finish just apply enough pressure and make a few up and down swipes with the grain. The oil will lubricate the piece and melt the residue you are making. Then just wipe off the piece afterwards. The sanding sponge works good for me because it fits into all of the nooks and crannies of the piece. You can always go back over some shiny spots in the piece afterwards if needed.

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Using the same product produced the same result shown - high gloss finish. After letting initial coat set up for 3 hrs, stirring the can well before and during application resulted in the desired semi-gloss finish. Just need to stir the heck out of it.

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