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Reply by CaptainKlutz

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Posted on Can I slow down the drying of shellac?

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CaptainKlutz

1490 posts in 1911 days


#1 posted 05-19-2019 12:00 AM

Yes. Shellac can be made to dry slower.

Can slow down evaporation rate with any finishing material, by using a slow evaporation solvent that is compatible with system. There are many references on web with solvent evaporation rates and compatibility, such as:
https://www.eastman.com/Literature_Center/S/SOL030.pdf
https://www.eastman.com/Literature_Center/R/RES001.pdf (shellac is considered a rosin ester in this chart)

IMHO – Most common retarders for spraying lacquer and urethane systems are Methyl N-Amyl Ketone (MAK) or Propylene Glycol Monomethyl Ether Acetate (PGMEA or PM Acetate). Common retarder for water based systems is Propylene Glycol (not to be confused with PGMEA).

If you have ever dealt with automotive spray paints, they offer secret blends of retarder solvents designed to work in narrow range of temperature/humidity. The solvent blend is designed to slow evaporation, but may also use solvents to reduce runs/sags with slower evaporation, or fix other issues created with retarding the finish. It gets complicated quickly, but once you get educated on how different solvents work, you can easily make your own retarder. Regardless, back to shellac…

Since shellac is dissolved in alcohol, petroleum mineral solvents (or Stoddard solvents) just aren’t compatible. Compatible means it will mix with other solvent(s), not turn cloudy, and doesn’t impact finish final properties.
What is compatible?
If you read the MSDS from qt can of Zinseer on Amber Shellac you find they use 2-Propanol and Ethanol for main solvents. The MSDS for the spray version they add n-Butanol, and acetone. Which just proves that you can use more than alcohols (ketone’s) in shellac. :-) Most ketone and some ether/ester solvents are compatible with alcohols.

Here in AZ desert low humidity, have to use extra solvent plus fast retarder in winter, and slowest retarder in summer; otherwise normal fast evaporation finish might dry before it hits the surface. I mostly spray de-waxed shellac dissolved from flakes (used as blotch control), and use n-Butanol and/or PGMEA as retarder with 90/10 ethanol/methonal as main solvents.

The PGMEA has nasty odor which forces full respirator mask all time, but is compatible ester solvent. Should always use respirator when working with shellac, but since the alcohol only makes you dizzy, and isn’t stored inside your internal organs for very long; most don’t think one is required as they drink their favorite whisky while applying shellac. :-)
Be safe with solvents, always read MSDS and use proper protective equipment (PPE)! If you dissovle your own flakes in hardware store alcohol which has poisonous methanol in it, always wear PPE!

Finding retarders or solvents can be challenging thanks to US government rules written by EPA and alternate uses monitored by DEA and Homeland. Rules are simple: Solvents must be handled, and disposed of properly by educated law abiding people, hence they are only sold at dedicated purpose stores that keep records of sales.
Can generally find retarders and solvents are any auto painting supplier, industrial wood working finish supplier, or laboratory supply (most expensive). Not ever brand store (PPG, SW) carries very type of plain retarder solvent, so be warned you may have to shop around. Do not forget to read MSDS as many branded retarders have product names/numbers and will not say that it is PGMEA, MAK, or common blend of PGMEA/MAK.

Best luck on your finishing solvent adventure!
Hope this helps.

PS – I am just another useless idiot posting information in a web forum that may or may not be 100% accurate. The above is simplification and may not agree with others scientific definition of retarder solvents.
So, YMMV

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!


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