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Poly not drying

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Forum topic by 67flh posted 02-12-2020 12:43 PM 501 views 0 times favorited 8 replies Add to Favorites Watch
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67flh

117 posts in 2528 days


02-12-2020 12:43 PM

Topic tags/keywords: question

I recently put a coat of polyurethane on a small white oak piece I was working on. In the past it was dry enough to sand within 24 hours. After 36 hours item was still to tacky and seemed to be stalled the finish would not dry. I took some mineral spirits and removed the poly, thought perhaps it had gone bad so I bought a new can. I re-coated the piece and must admit that it is a lot dryer after 24 hours than the first time I did it. It’s winter, air is dry and house is only around 68 degrees. Is this normal?

-- Brad


8 replies so far

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OSU55

2830 posts in 3236 days


#1 posted 02-12-2020 01:08 PM

Oil based poly? Brand and product? If you put it on thick, no it could take a couple of days to start firming up. Some products can have higher viscosity allowing thicker coats which just take longer to dry and cure. A thick film will take 30 days or longer to cure out.

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67flh

117 posts in 2528 days


#2 posted 02-12-2020 01:24 PM



Oil based poly? Brand and product? If you put it on thick, no it could take a couple of days to start firming up. Some products can have higher viscosity allowing thicker coats which just take longer to dry and cure. A thick film will take 30 days or longer to cure out.

- OSU55

Minwax Fast Drying Poly , the can says let dry 3-4 hours then sand allow 24 hours after final coat. I put on a thin coat. Thanks for your response.

-- Brad

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ChefHDAN

1828 posts in 4096 days


#3 posted 02-12-2020 03:04 PM

Brad,
Poly is a debated subject, so love it some hate it, My kids are just now approaching their 20’s and just about anything I built for the house I protected with poly. From your description, I’d look to the fine print on the can for temperature and humidity recommendations for the product, the data sheet in the link calls for 77*f and 50% humidity. FWIW, since you’re enjoying retirement and a pretty sweet flathead, I’d suggest you take a look to spraying. I have the cheapo Harbor freight HVLP gun that I shoot minwax polycrylic right out of the can and get great results, generally I can get 3 coats on in an afternoon and then rub it out with paste wax & 0000 steel wool. There are lots of products out there and many of the more Pro guys here are very qualified to advise, but I’ve yet to have a project finished with MW PC that has not finished very well

-- I've decided 1 mistake is really 2 opportunities to learn.. learn how to fix it... and learn how to not repeat it

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splintergroup

5877 posts in 2469 days


#4 posted 02-12-2020 03:06 PM

Usually quite sensitive to temperature and humidity. Colder definitely extends the dry time.

Another source of drying issues is an oil finish used underneath. If that hasn’t dried then the topcoat of poly can remain tacky for a long time.

View SMP's profile

SMP

4841 posts in 1152 days


#5 posted 02-12-2020 03:13 PM



Usually quite sensitive to temperature and humidity. Colder definitely extends the dry time.

Another source of drying issues is an oil finish used underneath. If that hasn t dried then the topcoat of poly can remain tacky for a long time.

- splintergroup

Yep, here in the summer when its 100 plus I have the opposite problem. Its tacky while rubbing it on and end up messing up the finish. In cold its way longer.

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CaptainKlutz

4906 posts in 2741 days


#6 posted 02-12-2020 05:13 PM

Oil based Polyurethane finishes cross link via dual method. Standard drying oil via oxygen and moisture induced cross linking.
Another cure requirement is the temperature has to be high enough to allow the vapor pressure of solvents to evaporate from the film too. The thicker the film, the longer it takes for solvents to be removed.

A cold/dry environment found in winter time can delay cure time dramatically. The requirement for 50% RH environment is required to achieve the cure rate listed on labels.

+1 finishing with Poly in hot or damp environment can also create issues.

Too hot and the solvents dissipate to quickly, the tack time is only one minute. Can reduce these issues adding some extra paint thinner or Stoddard solvents.

Too much humidity presents a different conundrum, as the finish will skin over and make it harder for solvents to dissipate, plus lengthen the cure time of underlying material.
So before you take your slow curing piece into bathroom and take hot shower to increase humidity, give the finish enough time for solvents to evaporate and get tacky naturally.

Hope the ‘polyurethane education’ helps.

-- If it wasn't for bad luck, I wouldn't have no luck at all, Doom, despair, agony on me… - Albert King - Born Under a Bad Sign released 1967

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67flh

117 posts in 2528 days


#7 posted 02-12-2020 06:51 PM

Thanks for all the replies, I shoulda read the fine print. I have used Poly for for a long time I guess not during the colder months though. I always hit it with 400-600 wet or dry put a second coat on that always dries faster than the first and hit with wet or dry and paste wax. Hard woods finish like glass.

-- Brad

View MrUnix's profile

MrUnix

8775 posts in 3446 days


#8 posted 02-12-2020 07:00 PM

I always hit it with 400-600 wet or dry put a second coat on that always dries faster than the first and hit with wet or dry and paste wax. Hard woods finish like glass.
- 67flh

I only do wipe-on poly (no more sags, brush marks or drips for me!!) and have the opposite experience – the first few coats always dry much, much faster than the later ones. On some woods, like red oak, the first couple of coats can be dry to the touch in 5 minutes or less. After about 4-5 coats, it can take as long as 15-30 minutes to dry to the touch.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

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