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kmealy

TGIF Cold weather finishing Tue. Oct 17, 2017

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TGIF Cold weather finishing

 

As fall progresses, shops get colder and we need to understand what finishes we can use in cooler shops.

 

Let’s remember, finishes cure in one of three ways

  • Evaporative (Spaghetti*) – these finishes cure by evaporation of their solvent.  And when dry, they can re-dissolve in the same solvent.   Examples: Shellac, Lacquer, Wax
  • Reactive (Tinkertoys*).   These finishes cure in two steps.  First the thinner evaporates, known as “Flashing off.” Then the finish molecules form long chains by the chemical reaction with oxygen, known as “polymerization.  Examples: Varnish of all types, drying oils such as linseed and tung
  • Coalescing (Soccer balls*) These finishes also cure in two steps.  First the thinner evaporates, then the larger molecules of finish, softened by the solvent, get close and bond to each other as the solvent evaporates.  Example: Water-borne finishes, where water is the thinner and glycol ethers are the softening solvents.

 

Flexner uses these analogies* to explain the differences

 

Before looking at each of these, a basic chemical principle is that heat effects chemical reactions.  The general rule for most reactions is that every increase of 10C or 18F degrees doubles the rate of reaction.  Thus every reduction of 10C/18F doubles the length of reaction.

 

Another thing to remember is that reactive and coalescing finishes can take several weeks to fully cure – long after they are dry to the touch.

 

So what about finishing in a cold shop:

 

  • Evaporative finishes – these will cure, though at a slower rate because the evaporation may be slower.   Air movement will help.
  • Reactive finishes – these will also cure, but also at a slower rate.  Again, providing fresh air (i.e., oxygen)will assist in getting the reaction to occur.  Just give it more time.  If you need to thin, turpentine, a slower flashing off thinner, may help to getting it to flow out and level a little better.
  • Coalescing finishes – these are the exception.   They don’t cure well at lower temperatures, typically 65F.  May vary by brand, but should not be your first choice in cool shops.

 

Of course, one option is to finish the item in your shop and move into a spare room in the house (with SWMBO’s approval) to keep the curing on track.

Edited by kmealy

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