Shellac sealer issues

[Barry Holman]

Until recently I had been using a lacquer based sealer with very few issues. Just started using a 1 # cut of shellac made with flakes. I put on the first coat followed shortly by a second coat. The surface after sealing is very rough. After about fifteen minutes I sand with 320, the surface is better but not nearly as smooth as before sealing. The sandpaper clogs very quickly. Am I not waiting long enough to sand back? Other issue is the surface after sealing looks terrible. I have read threads that say my sanding is not good enough before sealing. I have used naphtha to try to find where my sanding before sealing was deficient but I don’t see anything wrong. On both walnut and cherry that I did not seal I see no difference between sealed and unsealed finishes. Do some woods not need a sealer? Frustrating!

 

[Robert Santore]

It seems odd to me that your clogging sandpaper with a 1# cut. In my experience a 1# cut is very thin and soaks in completely and sands well. You might want to check your recipe to verify that it's not more concentrated.

Shellac can also get gummy when it's old, but I have never seen that be a problem if it's freshly made from flakes.

You can test it, though, by putting a little puddle on a piece of glass and letting it fully dry. Test the residue by scratching it with a fingernail and seeing if it forms a hard film. If it's gummy, you might just have a bad batch, but that could be why it's clogging sandpaper.

 

[Richard Coers]

If the humidity is high, “shortly” is probably too soon. There is a shelf life for flakes as well as premixed shellac.

 

[Tom McDonnell]

I sand shellac sealer at very low lathe speeds, or just by hand, off the lathe. High speed sanding will clog your paper.

 

[Doug Freeman]

Do some woods not need a sealer?

Don’t use sealer on anything, unless I’m spraying, which is usually solvent lacquer, and it self seals. What is your intended finish, and why are you using sealer?

Lacquer worked better as a sealer for hand finish application, due to your stated issues, and the cost of shellac has risen dramatically over the past 5 or so years. I dont like using lacquer much due to the volatiles. FYI Water based finishes can be thinned and used as a sealer. Some of the “oil modified” wb finishes may play ok with oil or poly top coats, not positive.

 

[Roger Wiegand]

Shellac melts when you heat it up and will clog sandpaper. Try sanding less aggressively, it should come off like dust, very easily.

 

[Kent Reisdorph]

Thanks for that, Roger. I'll keep that in mind.

Barry, I get the same thing in my high humidity environment. I typically apply shellac as a sanding sealer after 180 grit and before I go to 220 grit. I will hit it again at 180 after the shellac has dried, and then go on to 220. The 180 sandpaper disc gets gummed up in a hurry. After I have sanded again at 180, I go on up to 220, and 320. I don't see any more gummy stuff at 220 and later.

 

[Jaramiah Severns]

Thanks for that, Roger. I'll keep that in mind.

Barry, I get the same thing in my high humidity environment. I typically apply shellac as a sanding sealer after 180 grit and before I go to 220 grit. I will hit it again at 180 after the shellac has dried, and then go on to 220. The 180 sandpaper disc gets gummed up in a hurry. After I have sanded again at 180, I go on up to 220, and 320. I don't see any more gummy stuff at 220 and later.

The gummy texture is usually due to a) not giving the shellac enough dry time and / or b) too much friction due to sanding at high speed.

The former is usually more of an issue once the shellac has saturated the wood and begins to sit on the surface; 30 minutes usually does the trick, but the more layers build up, the more time it takes. If I’m going super high gloss with many layers, I’ll let it sit over night before sanding (and ALWAYS give it 24 hours to cure before buffing). To mitigate the latter, I never sand shellac at more than 200 rpm and tend to keep my drill at about half speed.

 

[Kent Reisdorph]

The gummy texture is usually due to a) not giving the shellac enough dry time and / or b) too much friction due to sanding at high speed.

The former is usually more of an issue once the shellac has saturated the wood and begins to sit on the surface; 30 minutes usually does the trick, but the more layers build up, the more time it takes. If I’m going super high gloss with many layers, I’ll let it sit over night before sanding (and ALWAYS give it 24 hours to cure before buffing). To mitigate the latter, I never sand shellac at more than 200 rpm and tend to keep my drill at about half speed.

Great advice from the Shellac King. Thanks, Jaramiah. I hadn't considered disc speed.

 

[Randy Anderson]

I use shellac, mixed from flakes, all the time as a sealer and as a finish on some pieces. I make it at a 1.5lb cut which seems to be a good balance to work as a sealer and as a finish. My experience is when it isn't working as I hoped it's because I've not given it enough time to cure. 30 minutes to an hour is the common advice you'll see but, it can feel dry but not be cured. When in doubt I go do something else for a while and come back. If you're sanding between coats doesn't produce fine powder then it's not cured. I never "spin" sand it. I either use a ROS sander on large pieces or my hand drill motor sander or a foam sanding block. Spin sanding seems to run the risk of generating heat or leaving sanding trails on my work.

As for buffing I use a very small amount of Yorkshire grit on a cotton cloth with light pressure and minimal speed.