shellac as undercoat

[sscoville]

I'm considering using shellac to seal my wooden masts before painting them.  I was going to coat them in epoxy, but I have some shellac and it seems this may be a good alternative in this application.

[PAR]

Shellac is much better then traditional oil finishes at resisting moisture vapor penetration and slightly better then traditional varnishes. It's was a common sealer before the advent of modern waterproofing materials.

This said, it's not going to stabilize the wood like epoxy will. Epoxy will stop movement (swelling or contracting) of the wood through moisture gain or loss, shellac will not.

On the other hand, shellac is a lot easier to repair and isn't going to trap moisture under it's surface, in the event of a breach, like epoxy can.

If you've used epoxy as the adhesive for your mast assembly, then you'd be best advised to continue using epoxy to seal and stabilize the wood, encapsulating it from environmental changes. If you've used a different glue to assemble your stick, then shellac has a good bit in it's favor.

[hokeyhydro]

I use shellac on the Birdsmouth mast and boom I'm building. All parts were coated with WEST resin and assembled with WEST. I had installed a few backer strips inside the spars where rigging parts are to be attached, and after assembly I squirted a cup or so of shellac inside the spars and ran the shellac end to end and around and around to coat the interior, allowing the excess to drain.  The spars exterior will be multiple coats of WEST with multiple coats of spar varnish.

[PAR]

I'm not sure why you felt the need for using shellac over epoxy, but it shouldn't hurt anything. Shellac is only about 75% effective at preventing moisture vapor penetration. West System epoxy is about 98% effective.

[hokeyhydro]

PAR - shellac over epoxy . . . Yes, WEST is good for moisture penetration, but WEST recommends SIX coats to achieve a good seal. And I added doublers inside the boom as an afterthought by tack gluing aforementioned doublers to a thin stick, slobbering epoxy on them, and sliding them inside the already assembled boom. There was a tad of bare wood left, and the interior parts had two coats of WEST so I added shellac for backup moisture resistance.  Probably overkill since both spars will be plugged to prevent flooding during a knockdown and the usual flooded mast turtle turn.

Answer to the original query: I don't use shellac as an undercoat. But I do use several coats of shellac as the only coat for workshop jigs and cabinets. Dries fast and smells good  :-)

[PAR]

Insure you have small drain holes fully through any plugs or hard points inside the the mast. You don't want to completely enclose any area inside a mast. You want a way for condensation to get out and this is what the drain holes offer. I use 1/4".

[hokeyhydro]

This is true = drain holes. 1/4" is a nice size. I plan to bung the holes while sailing since the Beach Cat pilot is a novice and I fully expect multiple capsizes before she figure s it all out. 

[PAR]

I wouldn't bother with these "underway" plugs.

As soon as you capsize, get in the habit of swimming out to the end of the mast and lashing a PFD to the mast top. This will keep it from going turtle and you can leisurely swim around, gathering up all the floating beer cans, before you right the boat again.