Polyurethane question?

Re: Polyurethane question?

JohnV

My experience with poly is that varying conditions of humidity and temp will greatly affect drying and curing times. For furniture I like to leave 48 hrs between intermediate coats and even longer before the finish coat. Even it will still be soft for up to a month.

My 2 cents.

Too soon for that second coat

Dick Coers

Your early application will likely extend the full cure by quite come time. No way to predict if and when.

Re: Too soon for that second coat

Barry Irby

The Instructions on the can said to recoat in about 3 to 4 hours and if you waited more than 8 hours to sand between coats. So I thought I was On the cusp of sanding or not sanding, so I sanded.

The other interesting thing was I was going to put on a coat of amber shellac to improve the color of the fuming. (A suggestion made along the way.) I bought a can of Zinzer Amber Shellac and then carefully read the label after I got home with the aid of a magnifying glass. It said not to use it under Moisture Cured Urethane. The Varithane can did not reveal whether it was moisture cured or not. The Amber shellac did not say it was Dewaxed so based on both of those things I was reluctant to use it. I also read the label on their dewaxed Seal Coat and it said not for use under Urethane. So I did not use that either. I also looked at Varithane's Sanding sealer, but it was water based and I was reluctant to use it because I wanted something that would pop the grain and enrich the color. I was also suspicious that sanding sealer would not be as tough as the oil based urethane.

Re: Too soon for that second coat

Don Stephan

Testing on sample pieces before committing can be reassuring as well as minimizing unfortunate results.

Re: Too soon for that second coat

Barry Irby

I did a test on several small pieces of the very same oak that had been fumed the very same way and it worked well. The weather was warm and dry during the test and cold and damp when I got to the table base. So the stuff was the same but the conditions changed.

cure time and temperature

Bill Tindall, E.Tn.

The cure time is not proportional to temperature; it is an exponential function. It could take weeks to cure at 55 degrees. 75 ok. If it looks good it is good. The only way it can go "bad" is to cure.

read directions on subsequent coats

Bill Tindall, E.Tn.

To get intercoat bonding subsequent coats are best made as soon as the previous coat feels dry. The previous coat must not be fully cured for the two layers to chemically bond.

You are correct that full cure could take a week or more depending on temperature.

Why....

Bill Tindall, E.Tn.

......did you feel compelled to use something between wood and varnish? No shellac necessary or advised.

Amen...

Bill Tindall, E.Tn.

.....but Barry don't work that way.

Re: Why....

Barry Irby

I had read that amber shellac improved the color of the fumed oak, adding warmth and depth. Some of mine had a grayish cast and whatever I read specifically mentioned improving the grayish color. I also fell for the old saw that Shellac sticks to every thing and everything sticks to shellac. And I was less than comfortable with have a little over a quart of finish for three coats. So I was thinking if the first coat were shellac, maybe the subsequent coats would go further and I would not have to use the urethane of the under side of the top.

Have I ever mentioned that in spite of my appearance of bumbling through life I may have a tendency to over think things?

Re: cure time and temperature

Barry Irby

Thanks, I was thinking that the business of re-coating quickly meant that while "dry" to the touch the finish would not have fully cured and the new wet layer would link to it, but if you waited longer you needed to scuff sand to provide a mechanical "tooth" for it to bond to.

The exponential idea had not occurred to me. Next time...There won't be a next time. It will be something different.

all amber is created equal

Bill Tindall, E.Tn.

Urethane varnish turns yellow with age and it is fairly yellow to begin with. Whoever advised that amber shellac was needed beneath seems to have failed to remember these facts.

Secondly, it makes no sense to undercoat a finish where you are striving for durability with one that is less durable. I'll bet with testing it will turn out that the impact resistance of urethane undercoated with shellac is less than urethane only.

correct

Bill Tindall, E.Tn.

"Thanks, I was thinking that the business of re-coating quickly meant that while "dry" to the touch the finish would not have fully cured and the new wet layer would link to it, but if you waited longer you needed to scuff sand to provide a mechanical "tooth" for it to bond to. "

"link" more properly called "chemically bonded".

On the other hand when the manufacturer advised 5 hours they were not thinking at 55 F.

Me neither....

Barry Irby

I have a crappy little barn heater in my shop. The thermostat on it has degenerated to the point it has about a 20 degree swing. Somewhere between uncomfortably cool and way too hot to work. Apparently when I left the shop I tweaked it down and landed somewhere around 50 or 55.

If I recall the can said something like between 50 and 75 degrees. I just did not expect to go quite that near the bottom. I actually meant to go back later and apply a second coat, but 9 pm has become the new midnight. So I went back this morning at 6 or maybe 7 AM. (Damn clock change) Where is all that daylight we saved now that we need it?

The Daylight

Mark Mandell - Gone Round In New Jersey

went South.

[thought you knew that ]

Wax or No Wax

David Yoho

Between the four usual Zinsser shellacs found in the Big Box stores, two contain wax and two don't. The cans of shellac marked Amber and Clear contain wax. Seal Coat and the spray cans of shellac are dewaxed.

Call me too careful, but...

Ellis Walentine

... I wouldn't have used old varnish on a big important project like this. And, I'm wondering about the aluminum oxide in the varnish. Isn't that formulated to make stair treads grippy? I would have applied two or three coats of fresh gloss varnish (these days I am liking Helmsman) and followed up with a final coat of satin. I did this on our kitchen counters about ten years ago and it has worn like iron.

Regarding the appearance of your fumed oak, Stickley himself played a lot with post-fuming techniques, including dye stains, spirit varnishes and banana lacquer, which was essentially amyl acetate, about as colorless a lacquer as you could find. Today, if you want to warm up the fumed oak appearance, the best way would be to mix up a stain that produced the desired look. And I agree with others that you've extended the cure time by recoating too soon, that warmer temps will speed up the curing, and that you don't need a sealer under urethane.

For what it's worth, when I was finishing large pieces during the winter, I would roll up the living room rug and bring the piece(s) into the house to finish them. (My shop was always heated with wood.)

Ellis

living room cure

Bill Tindall, E.Tn.

Curing, mistakenly called "drying", varnish is a two step process that occurs simultaneously. The quickest step is evaporation of the solvent. The slower step, and the one that requires high temperatures, is the reaction of the varnish components with oxygen to crosslink.

The solvent evaporation step produces copious fumes which some people don't like. The cure also produces smell from volatile chemical pieces that are byproducts of the curing oil in the varnish. Good rule of thumb, if it smells like varnish it is still curing and becoming more durable.

When it is cold I cure inside too. As soon as the finish is not tacky it gets moved into the house. At this point most of the solvent, and odor, is gone. Only the odor of a tiny amount of exceptionally stinky curing byproducts will be detected. Celebrate this odor as a sign that the furniture piece is done and near ready to use.

Re: living room cure

Kurt Kiesel

I have been reading all the wonderful knowledge that is so freely given on this forum for a couple of years and have never felt compelled to chip in. Probably because I would be like a kindergartner talking to graduate students but here goes. I to cure somethings in the house as Nebraska winters can be brutal, or beautiful often in the same day. I learned a hard lesson about where to allow this to happen, if the off gassing happens to find its way to your hot water heater you may find yourself taking a cold shower. Several years ago I finished a small project in the basement of my home and almost immediately began having water heater problems. Turns out they have a sensor that I suppose is designed to keep a person from blowing themselves up in the event of a gas leak and they can be sensitive to about any type of fumes. Luckily my plumber is a friend and busy enough to walk me through the fix and wholesale me the sensor so a couple of buck, a few cold showers and I learned a lesson.

Making a decison on "old" varnish

Bill Tindall, E.Tn.

Varnish is a mix of solvent, resin, crosslinking oil and catalyst (and delusterant ). What could go wrong in time? What would wrong look like?

Solvent could evaporate. Varnish would be obviously more viscous. Remedied by adding more solvent. Never will happen in a metal can.

Resin- absent oxygen or light unlikely any chemical change.

Oil- again, absent oxygen or light unlikely any chemical change.

catalyst- stable for ever

air/oxygen leak- varnish will cure and separate from solvent as some sort of suspended solid gunk.

Delusterant settled- might not be able to re-suspend. Fatal.

But if there is any lingering doubt, apply some to a test piece. If it hardens in an expected amount of time its as good as new.

I have no reservations using "old" varnish because it tells me if it is still good either by not showing separation or by a test application. The best reason to use old varnish is the proclivity of companies to reformulate. The new stuff my not perform like the old stuff. Example, I have never found a replacement for Sherwin Williams "Beautylock". I nursed a gallon for at least 25 years before I ran out.

volatile flammables is volitile flammables

Bill Tindall, E.Tn.

Volatilized mineral spirits is just as flammable as methane. If this sensor accurately detected an explosive concentration of gas or solvent you were at risk. Note I said I cured the furniture indoors, as separate from the solvent volatilization step done in a well ventilated area.

I spray lacquer outside when it is below freezing. As soon as solvent is flashed the piece comes indoors to cure. No flammability hazard and little odor.

Good explanation

Ellis Walentine

I just never know when the next finishing shoe is about to drop, so I play it (sometimes overly) safe. I recall too many cans of old varnish, Waterlox etc. that had started to gel and or had changed in ways I couldn't fathom. Maybe part of it was from experience with shellac which had started to esterify after some expiration date. Rather than take the chance, I'd write off the cost of fresh finish to the job and not have to wonder or worry about it.

Ellis

Any hint of gelling-fatal

Bill Tindall, E.Tn.

Anything starting to gel is not to be used. A multitude of problems could result. Gelling is an indication of an oxygen leak and oxygen can lead to many decompositions and polymerizations which could affect appearance or durability.

Shellac- harder to tell when it is unfit to use and I have little experience dealing with its personality.

Re: Any hint of gelling-fatal

Barry Irby

I think I said this before, but I opened the can of Varithane to check it out with every expectation it would be ruined, some sort of glop in the can. Not so, it was like new, no scum, no thickening, no sediment. I was surprised. I coated two test pieces with it and it cured exactly as I expected. So I have used it.

The first coat did not cure well or thoroughly, but Bill pointed out that I was correct in pointing to the temperature in the shop, probably around 55 degrees. Since then I have turned the temp up to unpleasantly warm, probably around 75 and it has cured nicely.

If there are any aluminum oxide particles in the mix they are too small for me to see. Must be very fine. Also can't feel them on the surface. There has been no sediment in the bottom of the can, even after it sat for several years. I stirred it gently to avoid incorporating bubbles but found nothing in the bottom.

Had there been any degradation of the finish or the test pieces did not cure as I expected I would not have used it.