Shellac Prior to Varnish?

Steve Ambrose

Nut in a Canoe
Looking for guidance/input on the use of Shellac to seal the wood and build up a few coats prior to applying a final coat or two of varnish. Pros and cons?
Shellac is a beautiful relatively old finish and is often (a bit too often in my estimation) used as a sealer for wood. It is wonderful for sealing off things that might react with varnish like silicone, oils, resins, waxes, stains and odors in the wood ( it covers smoke and animal odors very nicely).

However, unless your boat is made from a resinous wood like pine, a herd of cats used it for a litter box or is in the process of being refinished and a stripper was used to strip the old finish, I'm not convinced it's the best solution in all cases. I think it is best to stick with one finish for all your coats unless you are likely to have one of the problems mentioned above. Mixing finishes produces weaker bonds between the finishes and can lessen the protection and durability of the primary finish (top coats). Especially if the primary finish is more protective and durable than the shellac...which most varnishes are. Shellac that contain wax does not bond well with polyurethane so should not be used under that type of finish. Zinsser makes a de-waxed shellac that could be used if need be.

Having said all that, I do use shellac on many of the items that I build. All of the guitars I make are French Polished, often my pine furniture is sealed with shellac, it's also the main finish for many of my other furniture projects and is also my base coat on any canoes and wood boats that I re-finish. However, on the new boats I don't seal the wood with shellac unless I anticipate an issue with one or more of the problematic substances mentioned above.

On new wood I usually follow the manufacturers directions for thinning which ever varnish I'm using (if necessary), use the first coat of varnish as a seal coat, let it cure well (two days, sometimes less depending on conditions), sand it well and continue with my finishing routine.

As I said, I think is an awesome product and unfortunately it is often touted as a sealer for all occasions. It is not but is a valuable tool when used in the correct situation.
Thanks Scott. I have two boats that are close to the varnishing stage: a 1937 OT Guide with lots of new wood mixed with the old, stripped, stained, and oiled. Plus a new Cheemaun. I've never used Shellac but have been reading various posts here and wondered if it might be appropriate. I really don't want to experiment after all the work I've put in only to have to strip and refinish :eek:

Guess I'll stick with spar varnish for the interior. Any thoughts on using Shellac on the outside of the hull as a sealer prior to canvassing? I hate to use the high dollar varnish just to seal the outside of the planking but certainly don't want to use incompatible finishes resulting in problems under the canvas!
Your OT Guide refinish is the kind of situation where I would use a coat, maybe two, of shellac as a sanding sealer. I just think it gives the varnish "level ground" to uniformly adhear to. You've got a number of things going on there, stripping and and residue left over (however minute) stain etc. Others might not feel the need but I just feel more comfortable doing it.

The Cheemaun, being new, is not where I would usually use the shellac. I would just keep to one finish there.

As far as the outside of the planking, on both a new build or a re-do, I like to give a coat of diluted varnish after oiling it and before finishing the interior. If some dribbles through to the interior, and it seems it always will (it's just the universal law), it's easier to clean up the drips than to have to fix a perfect interior finish...I learned the hard way. And any drips from finishing the inside to the out don't matter as they can be easily cleaned up and will be covered by canvas anyway. I don't use the expensive stuff for that, I don't think it's necessary. You just want some moisture protection for the planking.

For the most part shellac is relatively easy to use. It doesn't have the same leveling properties as varnish so takes a bit of getting used to as far as applying it. Although from my musical instrument finishing I can say that I see it level out over a few weeks or sometimes months. But I'm sure you wouldn't want to wait for that to take place on your canoe.

When brushing it's best if applied with smooth quickish strokes, usually in one direction with minimual overlap. Being alcohol based it dries very quickly and conversely is easily sofened by the same alcohol so by going over the spot of recently applied shellac with a wet brush one can lift the first coat off. But don't let that frighten you. A little experimentation and you will find brushing it for the purposes we are talking about here very easy.

Now brushing furniture grade finishes or French Polishing shellac there is a whole other lifetime of experience necessary. I've been making my own shellac mixture and French Polishing since the early 1980's and most of the time I feel I'm not much better than a beginner. It's an alchemists art, for sure.