Some questions about shellac

[JSpill]

I plan to use shellac to finish a mantle clock. I have a few questions. Does an open container have a shelf life? I haven't used shellac from this container for several years. Do I lightly sand between coats? How many coats are needed?

[Doug_H]

Jim,

Shellac does deteriorate in the can after opening. Sometimes old shellac won't dry. That's the only deterioration I know of from age. It's easy to find out if its good -- just brush some on a piece of wood and give it 30 minutes. See if it dries hard. Sand with 180 grit or so. It should produce fine dust with no gumminess.

As many coats as you like. I would say at least two. The first coat will raise the grain slightly. Sand lightly and evenly until smooth with 180 grit. The second coat will then dry smooth and nice. Additional coats will improve the appearance of figured wood.

The thing about shellac is that it dries, literally. Additional coats will dissolve the earlier coats somewhat. This is good for adhesion, but it means that you need to apply the coat and leave it alone. Don't go back and fuss with a wet coat. If you don't like it, let it dry, sand and apply another coat until you are happy.

Doug

[Yetloh]

I agree with Doug's comments although I have never bought it ready made up in a can. I buy shellac flakes and make it up with alcohol, that way you can make up as little or as much as you need fror the job in hand.

Jim

[JR1]

I agree with Doug and Jim! Apply some shellac to scrap—any wood will work—if it doesn't dry in ~20minutes (normal room temperature) it's no good.

Always sand shellac by hand with the grain of the wood. A light sanding with 320 between coats works for me and I normally don't sand or rubbout the final coat.

[Bill Wilson]

Couple other relevant points about shellac, Sanding between coats isn't necessary to get good adhesion, like it is for polyurethane. As was mentioned, shellac melts into itself, so sanding in between is to achieve smoothness. The degree to which sanding is necessary and when to sand can be left to your discretion.

Also, it's my understanding that you do not want a thick build with shellac, so the number of coats will depend somewhat on the cut of the shellac. The thinner the cut, the more coats you may need, but stop when you get the look you want. Shellac is a hard, brittle film finish, so applying it too thick could lead to cracking, crazing in the future.

[Yetloh]

Cut apart, I think the number of coats is a matter of finding the right compromise between wear resistance and the problems Bill refers to regarding chipping etc. I generally use a 2lb cut and for something that is not subjected to wear and tear (some wall lights I made recently come ot mind) I find that three wiped on coats are sufficient. I'm currently making a set of four ash dining chairs which will get much more handling, and these will have about six wiped on coats.

However, one of the real joys of shellac as a finish is that it is so easily repairable. Even an over-thich chipped finish could be repaired by stripping back the damaged area locally with alcohol and then applying fresh shellac which will partially melt the old finish and blend with it. It is even easier to repair an area that has worn through with handling.

Jim

[JR1]

Shellac is IMHO the easiest finish to repair. In many cases just wipe with a rag and alcohol. I find that the durability of shellac is not much affected by the thickness of the shellac (cut or number of coats) but if an object is going to be routinely handled then the shellac should be topcoated with a high durability finish. The problem with applying lots of shellac is sag which is difficult to fix.

[bhh]

This article, that came in this week, will give some more information. And yes, open or not, shellac will degrade over time.

http://360woodworking.com/bad-shellac/

[JR1]

There is another problem besides ot of date shellac and that is Zinsser has no control over the conditions that shellac is transported or stored in, and you have no way to find out. Mixed shellac stored at high temperatures can go bad quickly. This is why you buy flake and mix as needed.

[jteneyck]

I've never had a can of SealCoat go bad and some were over two years old. I have had a couple of cans leak, however.

John