TGIF: Three things that affect finish curing Tuesday, March 20, 2018

[kmealy]

Back from  a couple of hectic weeks.   I'm working on the presentation of a day long wood finishing seminar that I'm giving in April.   As I do the outline, it occurred to me that most finishing things occur in threes.

 

There are three things that affect how a finish dry or cures.

 

1. Temperature

Some finishes, like waterborne, don't cure well below a critical point (mid 60s for example). 

 

The general rule of reactive (Tinkertoy) finishes is that chemical reactions double in speed for every 10 degrees Centigrade (18 Fahrenheit).  So  a varnish or oil finish will cure about twice as fast at 78 degrees as at 60 degrees.  These finishes cure by absorbing oxygen and reacting with it to form polymer chains.

 

For evaporative (spaghetti) finishes, lacquer and shellac, they cure by evaporation of their solvents.  Higher temperature means faster evaporation.

 

2.  Air movement

Waterborne finishes cure in two stages - evaporation of the water (a thinner) then by the evaporation of the glycol ethers.

 

Varnishes also cure in two stages, first evaporation of the thinner (mineral spirits), then reaction with oxygen.  Oils and varnishes then cure by reaction with oxygen. Air movement speeds the evaporation of the mineral spirits, then makes oxygen available.

 

Air movement speeds the evaporation of the solvents in shellac and lacquer.  

 

When I'm doing touch up work, I'll usually use a hair dryer to speed the drying of the lacquer (adds both heat and air movement).

 

3. Humidity

High humidity will take longer for the water to evaporate from waterborne finishes.

 

Humidity, I believe, will have minimal effect on varnish and oils.   Since mineral spirits and water are not miscible (will not mix with each other) it should not change its evaporation rate.   And the cure rate should be irrelevant.

 

Shellac's solvent is alcohol, that is miscible in water (think cocktails or beer) high humidity will slow its evaporation rate

 

Lacquer presents another problem.  High humidity can cause lacquer to "blush" -- develop a white, cloudy appearance.  Because lacquer dries so fast, high humidity does not allow the water to evaporate out before it cures.  The solution to this is to use or add a slower-evaporating lacquer thinner to allow a longer dry time.

Edited March 19, 2018 by kmealy

[lew]

Thanks!

[PostalTom]

I have been thinking of getting a new clock for my shop,  and I am considering one that has both temperature and humidity dials on it.  Do you think this is something worth having, both for finishing and joinery issues, or are those just bells and whistles that aren't really all that useable?

[kmealy]

1 hour ago, PostalTom said:

I have been thinking of getting a new clock for my shop,  and I am considering one that has both temperature and humidity dials on it.  Do you think this is something worth having, both for finishing and joinery issues, or are those just bells and whistles that aren't really all that useable?

I think it depends on where your shop is and what type of climate control you have.   It's different between New Orleans, Santa Fe, and Duluth.  And if your shop is not heated / air conditioned in the months where it would be useful.

Edited April 10, 2018 by kmealy

[PostalTom]

I'm in Wichita KS,  and my shop is in my basement.  The water heater and AC/furnace system are in the room next to my shop, separated by thin paneling nailed to 2x4s, but no real wall with drywall, insulation, etc.  Laundry facilities were in that room, but we moved them upstairs several years ago to save my wife from having to use the stairs.

[Danl]

Postal Tom, what you are asking about is what I have in my shop.  I has time, temp, humidity, day, date, month, etc.  I can monitor the temp and humidity in two location.   Purchased from Menards when it was on sale.  Danl

 

[Stick486]

[Gerald]

I cannot see what good humidity would do . Temperature can affect both finishes and glues so that might help as far as cure time . Humidity would be a factor with lacquer but if the basement is on the central not a big deal . IMHO.

[PostalTom]

10 hours ago, Gerald said:

I cannot see what good humidity would do .

I was asking about humidity based on Keith's discussion of it's effects. It sounds like humidity is a factor, but if it's effects are minimal,  maybe I won't worry about it.  I guess my question was that since I need a new clock for my shop anyway, should I go the extra mile and include the temp and humidity indicators.  Thanks all, for the input.

[Gerald]

3 hours ago, PostalTom said:

I was asking about humidity based on Keith's discussion of it's effects. It sounds like humidity is a factor, but if it's effects are minimal,  maybe I won't worry about it.  I guess my question was that since I need a new clock for my shop anyway, should I go the extra mile and include the temp and humidity indicators.  Thanks all, for the input.

Tom my comment on humidity is based on the basement shop being on the home central system. If it is not then yes it would be a factor.