question about polyurethane - Woodworking Talk

phinds · 03-30-2009, 08:22 PM

I've been using various forms of polyurethane (all brush/wipe on, not spray) for several years and I find that there is ALWAYS the problem that no matter how quickly I pour some out and reseal the can (and I do that maybe 2 or 3 times over a month or two) what's left in the can always goes bad within a couple of months (hardens), whether it's just a bit at the bottom or most of the can still full.

Is this something others have experienced, and is there anything that can be done about it? (Yeah, I know, "buy a smaller can", but that doesn't really work for me).

Thanks,

Wood4Fun · 03-30-2009, 08:52 PM

I've got a couple quart cans in the garage that I've had for at least three years, and I have not even gotten a skin on the top. My garage/shop is only partially insulated, so it gets very hot during the summer and veeeerry cold during the winter.

something seems whacky if you got a whole can going bad that quickly. Maybe its a brand/manufacturing issue, do you have the same problem regardless of the brand? The stuff I have right now is Minwax, so nothing special.

I did have a mason jar of the Minwax mixed with spirits 50/50 that hardened after about a year, but there was only about a half inch in the jar.

Julian the woodnut · 03-30-2009, 11:31 PM

The only time I have had issues is when it's mixed with thinner and BLO to make a wiping varnish. I only mix it up in batches I will use up right away and don't have the problem any more.

I have been playing around with shellac/beeswax finished lately for some furniture and have fell in love with it. I really am not a fan of poly for fine furniture.

phinds · 03-31-2009, 10:53 AM

Quote:

Originally Posted by Wood4Fun

I've got a couple quart cans in the garage that I've had for at least three years, and I have not even gotten a skin on the top. My garage/shop is only partially insulated, so it gets very hot during the summer and veeeerry cold during the winter.

something seems whacky if you got a whole can going bad that quickly. Maybe its a brand/manufacturing issue, do you have the same problem regardless of the brand? The stuff I have right now is Minwax, so nothing special.

I did have a mason jar of the Minwax mixed with spirits 50/50 that hardened after about a year, but there was only about a half inch in the jar.

Hm ... Minwax is what I use as well. Both spar and regular in various levels of gloss, I always get the same reaction and you don't. Weird. I get more than just a skin, it starts to harden substantially. It's stored in a moderatly cool place (basement), never gets very warm or very cold. I've tried putting fresh batches in small containers, well covered, no air, but it doesn't matter, I get the same heavy skin after just a couple of months max.

It's REALLY annoying, as I use a lot of it over time, but end up throwing out easily half of what I pay for. Wonder what the difference can be?

Thanks,

jerry · 03-31-2009, 12:06 PM

Varnish is a reactive finish. It cures when it reacts with the oxygen in the air.While there are heavier than air products that help to lengthen the useful life of varnish, personally I buy varnish in quart cans.

Jerry

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bzbatl · 03-31-2009, 02:13 PM

I just cracked open my can of Minwax gloss poly yesterday. It's half full and had been sitting for about 2 months. Not even a skin on top.

Are you sure you're closing the can lid tightly? Rubber mallet FTW!

CivilEngineer13 · 03-31-2009, 03:38 PM

I use Minwax as well. I never have gotten the skin on top, but I have had some problems after a long period of time with the particles settling on the bottom and not being able to get them mixed up again. Don't know if this is normal or maybe I just give up too soon when stirring?

Gene Howe · 03-31-2009, 04:53 PM

I store the cans upside down. Don't know if that's the reason but, I get no skinning.

Julian the woodnut · 03-31-2009, 05:31 PM

Maybe you are getting contamination in the can that makes it skin over. Do you dip your brush in the can, or pour it into a seperate container?

phinds · 04-01-2009, 05:07 AM

I used to dip brush, saw that it caused contamination, don't do it any more. I open can, dip out enough to use (using a wax-paper cup), close can VERY tightly. This all takes maybe 30 seconds. I do this maybe twice over a month and the 3rd time I open can, it has a skin and the next time I open, it has a thick skin and is useless.

I've also seen the problem mentioned of particles settling to the bottom. I once bought a can where when I opened it, the bottom 1/3 was already sludgy. I skimmed off top 1/3 took can in and got my money back. Saw same problem again another time.

I love the way satin-finish makes my bowls look and the UV protection in spar poly is necessary w/ my exotics, but jeez this stuff gives me grief. Can't figure why I have more problems with it that others seem to.

firehawkmph · 04-01-2009, 10:14 AM

Phinds,
Maybe you could try this:
http://www.hartvilletool.com/product/11237
Mike Hawkins

creative novice · 04-01-2009, 05:52 PM

i'm perplexed as to why u and most of us don't seem to have this problem......mystery....sooooo......

do you purchase poly at a store that sells it fast enough or could it be old before you buy it? was it stored properly at the store or is it allowed to freeze/bake at the store? (buy fresh)
do you stir your poly well before you first use it? goop on bottom is part of the poly! stir it really wellllllllllll....stir again and again and again and then again before you put the lid back on it.
do u leave lid off and take a long time to use it while lid is off?....(use another container)
do you keep the lip clean so the lid seals well? (need a good seal)
do u punch holes in the lip as some do with house paint cans? (don't, if you plan on re-using the can)
could it freeze or bake where u store it?
do you add back that which you placed in a cup? (don't, esp if you took a long time to use it)
is the leftover poly less than a third of the can? (you can take seran wrap and place on top of the level)
did you thin it? (don't, thin in a separate container)
do you pound the lid back on properly?
do you have a neighbor who buys old stuff at yard sales and sneaks into your garage at night and switches yours out?
does your significant other resent your hobby?

then if none of these are the solution...maybe you should try beeswax!!!!!

sorry, not trying to be smart....just mystified!

Bob Wingard · 04-02-2009, 11:15 AM

Two things I have done to prevent this problem .. ..

1) Cut circles out of waxed paper the size of the ID of the can .. drop one on top of the remaining finish to keep the surface from contact with oxygen ..

2) Just before closing the can, hold this lid ALMOST shut .. then open the valve on a propane torch a bit and put the tip in the can, allowing some propane to flood the can. Propane is heavier than air, and it displaces the air, keeping what little is still in the can at the very top.

These two tips have allowed me to keep various finishes for extended periods of time.

<<<__ Bøb __>>>

phinds · 04-03-2009, 08:09 AM

Mike, thanks for that link to the bloxygen --- sounds like a winner

Creative Novice, I am careful about everything you point out, except that the stirring; I'll pay more attention to that. Thanks for the suggestions.

Bob, both are excellent suggests, although the wax paper removal prior to next use sounds messy. :-)

Thanks all. Still not sure why I'm having the problem and you're not (unless it IS the lack of stirring), but good suggests for ways of dealing with it.

Rick C. · 04-04-2009, 08:10 PM

phinds,
I too was having this problem.I either was told or read somewhere to stir poly 100 times in one direction then 100 the other.Seems like that would just put everything back where it was.Since then I no longer have the problem.I also started using a plastic lid with a smaller screw on cap(available at big boxes).This seems to seal better.
Good luck getting this figured out.
Rick

woodnthings · 04-04-2009, 08:54 PM

I personally use the paper plate or similiar size disc wrapped in a plastic bag for 5 gal paint cans and a smaller size for gallons and quarts to reduce the exposure to air/oxygen trapped in the can when you replace the lid. I always blow a big breath of CO2 into the can before I quickly replace the lid. From Wikipedia: At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.5 times that of air.
I saw this done somewhere and thought it made sense, but I'm no chemist. So it could be a wife's tale for all I know?

bill

phinds · 04-05-2009, 07:30 AM

Thanks Rick. It's really sounding like my lack of stirring IS the problem. I just bought a new can yesterday, and will crack it next weekend ... I'll try it and let everyone one know in a couple of months.

Thanks all,

phinds · 04-05-2009, 07:32 AM

Quote:

Originally Posted by woodnthings

I personally use the paper plate or similiar size disc wrapped in a plastic bag for 5 gal paint cans and a smaller size for gallons and quarts to reduce the exposure to air/oxygen trapped in the can when you replace the lid. I always blow a big breath of CO2 into the can before I quickly replace the lid:
At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.5 times that of air.
I saw this done somewhere and thought it made sense, but I'm no chemist. So it could be a wife's tale for all I know?

bill

Bill,

Don't know what's going on w/ the embedded links in your post; the mouseover shows what appear to be specific internal links but a click on either just gets me to a top-level forum page, not what the mouseover shows.

woodnthings · 04-06-2009, 03:55 PM

That particulat text I usurped from Wikipedia and those are their own links that just copied over with the text. I have deleted them and will from now on. Not a problem with this site or your computer! Thanks, bill

KE6WNH · 04-06-2009, 07:34 PM

I haven't had that problem with Watco Danish oil... OTOH Danish oil is purposely formulated to act more like a slow-drying surface penetrant than a simple outer coat. But back when I still used Varathane, some cans would go bad, while others were good for years. Go figure.