Shellac curing time?

[Joe McMahon]

I built a box for horse bridals as a gift. It is baltic birch with QS White Oak trim. I brushed on two coats of garnet shellac from a can. I want to use Arm-R-Seal as the top coat. How long should I wait for the shellac to cure before I apply the Arm-R-Seal? The project is in my heated shop at about 75 degrees.

[glenn bradley]

Around here it will dry to touch or re-coat in about 30 minutes. If I am going to top coat shellac I wait till the next day although many foils say 4 hours is enough.

[Stan Calow]

I just used shellac on a box project. I think from now on I will always give it at least overnight after each coat. Sometimes it seems dry to the touch, but I'll still leave fingerprint marks if I touch it after only a few hours. Maybe temp/humidity related.

[John TenEyck]

If you can still smell alcohol it's not dry. When you can sand it w/o the sand paper making corns it's dry enough to topcoat. I spray #2 cut shellac and it's dry in less than 30 minutes and I can sand it in less than an hour. 3# shellac that's brushed on is likely going to be heavier and take longer. #1 that's wiped on might be faster.

John

[Jim Becker]

Shellac doesn't "cure". It's an evaporative finish...it's "done" as soon as the alcohol flashes off. So you can coat in anywhere from a few minutes to a few hours. Shellac should never be applied thickly or built up thickly, either. Be sure you are using wax free if it's going under anything that contains polyurethane resins if you want proper adhesion.

[glenn bradley]

I just used shellac on a box project. I think from now on I will always give it at least overnight after each coat. Sometimes it seems dry to the touch, but I'll still leave fingerprint marks if I touch it after only a few hours. Maybe temp/humidity related.

Just in case this helps. If your shellac is slow to harden it is probably too thick. Multiple thin, thin coats is the key to happiness with shellac for me.

[Prashun Patel]

Try sanding it lightly with 320 or 400 grit sandpaper. If it does not gum or corn up but instead gets dusty and feels smooth after , it’s ready to top coat.

If you are wiping a top coat on, this is a fine test.

[Mike Henderson]

Try sanding it lightly with 320 or 400 grit sandpaper. If it does not gum or corn up but instead gets dusty and feels smooth after , it’s ready to top coat.

If you are wiping a top coat on, this is a fine test.
I agree with Prashun. While it "should" be ready as soon as the alcohol evaporates I've found that it takes more time to really "set". Prashun's suggestion of sanding (I use a stearated sandpaper) and seeing if you get corns built up on the sandpaper is good. Corns are small buildups of shellac on the sandpaper. if you don't change paper or remove the corns by scraping them off, they will make swirl marks on the rest of the finish. [Note: some sources call it corms instead of corns.]

I like to let a shellac finish go at least overnight before doing anything to it.

Mike

[off subject, but 5 minute epoxy is also not fully cured in 5 minutes. It takes 12 hours, or more, to fully cure. If you try to sand it off the work it will gum up before it is fully cured.]

[Prashun Patel]

I would also add that 'sand' means hand sanding. An orbital sander will can generate enough heat to cause an otherwise dry enough shellac coat to gum up.

Here's another lesson I learned the hard way:

if you are using shellac as a barrier/seal coat, you only need a coat or maybe two of thinned shellac. Resist the urge to put on too many coats - especially within a day. Subsequently topcoating that kind of surface with a varnish or other top coat too quickly can lead to problems like bubbling or blistering.

What I think happened in my case was even though the surface appears dry and not tacky, there is a small amount of alcohol in the surface. It makes its way out if the subsequent coat is thin enough. However, the thicker you make that coat, the longer that takes. Putting a thicker varnish coat too quickly on top of all that will really seal the alcohol in temporarily. However, when the surface is heated - like in the sunlight, the alcohol can expand and try to make its way through the harder films. This I believe caused the blistering. I am not sure if that's the reason, but it's what happened to me, and what I was told was the reason. YMMV.

[roger wiegand]

If your shellac isn't drying to a cleanly sandable state in an an hour or less it's either way too thick, or more likely, too old. Lots of the pre-dissolved stuff is too old when you buy it and stays tacky way too long. I always check my shellac before using it, if it doesn't dry hard in 15-30 minutes I pitch it and make fresh. The solvent can make a difference as well. I've had problems with some cans of DNA from the Borg, Benkhol (now Mohawk Shellac reducer) has always worked well and given longer shelf life of the dissolved shellac.

Most old shellac does eventually harden, and seems to be fine once it does. It just takes too much patience (not to mention accumulating dust nibs as it sits) and adds uncertainty to finishing, always a fraught process for me anyway.