Varnish Cure Question--Humidity v. Fresh Air Flow?

[Andrew Helman]

Which is more important for curing after the initial solvent flash off for varnish--low humidity or lots of fresh air and air movement? Things are dry to the touch but can be dented with a finger nail. I can have low humidity with a dehumidifier (dropped the RH from 75% to 45-50%) or lots of fresh air with a box fan in the windows--but not both because it continues to be humid where I am located. (And the AC didn't get the humidity down in any appreciable way.)

[John TenEyck]

If you are talking about traditional oil based varnish, air flow is more important than RH because those products cure by oxidation. But you don't really need to open the window. Just run a fan so it blows some air over the work piece while the dehumidifier is running also. There's plenty of oxygen in the air to satisfy the cure requirements.

John

[Andrew Helman]

Yes, traditional oil based varnish. Gotcha; thanks. That's what I thought; I appreciate the confirmation.


If you are talking about traditional oil based varnish, air flow is more important than RH because those products cure by oxidation. But you don't really need to open the window. Just run a fan so it blows some air over the work piece while the dehumidifier is running also. There's plenty of oxygen in the air to satisfy the cure requirements.

John

[Randy Goodhew]

Which is more important for curing after the initial solvent flash off for varnish--low humidity or lots of fresh air and air movement? Things are dry to the touch but can be dented with a finger nail. I can have low humidity with a dehumidifier (dropped the RH from 75% to 45-50%) or lots of fresh air with a box fan in the windows--but not both because it continues to be humid where I am located. (And the AC didn't get the humidity down in any appreciable way.)

Note: Fresh air movement does not increase the rate of polymerization of oil-based varnishes. The tiny amounts of oxygen removed from the ambient air during the curing process are rapidly replaced by simple molecular movement. Besides, air movement over an undried/uncured finish might move unwanted airborne particles and dust that could contaminate your finish.

Blessings.

[Art Mann]

I do a lot of spraying polyurethane varnish on small carved wooden items. My experience has always been that good air circulation and low humidity improves cure time. The manufacturer of the product confirms my experience. I just looked up the usage instructions for aerosol Minwax polyurethane. Here is a quote from their website.

"Note: Above dry times are based on good ventilation, temperature of 77°F and 50% relative humidity. Lower temperature, higher humidity, lack of air movement or application of thick coats will extend drying times."

I am pretty sure they know what conditions affect curing time for their products.

[Howard Acheson]

>>>> I am pretty sure they know what conditions affect curing time for their products.

Yes, I would expect so.

Flexner, other finishing gurus and finish manufacturers will all agree that oxygen is the component that determines the time necessary for oil based finishes to cure. Add more oxygen and you will get a faster cure. That said, finish manufacturers formulate their product to perform satisfactorily in a wide range of environmental conditions. I doubt that the expense to add more oxygen to the finishing environment would not be worth whatever small speed up in curing would occur.

[Howard Acheson]

Here is an article by a experienced professional finisher that fully answers to question. Go to http://askhlm.com/Articles/ArticleViewPage/tabid/75/ArticleId/32/Finishing-in-a-Cold-Shop.aspx

[Andrew Helman]

Thanks. In general, I've found that website to be very helpful. I ended up waiting--but I also used a dehumidifier for a week on loan from a friend. That upped the temp, dried the air, and added more air flow on top of the box fan I had going. I figured I'd cover my bases. It took about 26 days, but the stuff hardened up sufficiently for my needs--it's not epoxy, but it's good. I'll report back when completed.


Here is an article by a experienced professional finisher that fully answers to question. Go to http://askhlm.com/Articles/ArticleViewPage/tabid/75/ArticleId/32/Finishing-in-a-Cold-Shop.aspx

[Prashun Patel]

your dehumidifier will cause decent airflow in the room. You will have enough oxygen for curing unless you are finishing in a hermetic closet.

That being said, the ventilation is more important to your lungs.

[Jason Roehl]

Higher temperatures increase the evaporation of the vehicular solvent, shortening drying times.

Better airflow reduces the localized vapor pressure of the solvent, shortening drying times.

Higher humidity increases the localized vapor pressure at the workpiece, preventing off-gassing of the solvent, lengthening the drying time (unless it's a moisture-cure finish, which some polyurethanes are).