Thread: Sealing plywood query, Epoxy/Shellac

REDX,

you have used WS epoxy for the edges , just do 1 coat over your plywood faces and then , prime with the epoxy primer (I take it that this is a paint primer) .

Shellac is fine for furniture and may have been used on boats many years ago, however straight epoxy saturation coat or the products mentioned in Para 3, will do a better job, if you have a piece of furniture with a shellac finish , and you leave a glass with a wet bottom on it , you will see that the water will penetrate the finish and leave a white mark..

you can also use a product called EVERDURE or NORSEAL, these products soak in and seal the timber, and harden , they are similar to EPOXY RESIN, and give a good base for painting , or just sealing the surface against water ingress.


Jeff

The issue can get clouded by the numbers. Shellac is better then most paints, varnish and even some laminating resins, but the problem it two fold. The first is it's solubility in alkaline solutions and organic solvents, such as ammonia and alcohol products, but more importantly is the moisture vapor penetration resistance.

To positively seal wood, which makes it impervious to rot and stabilizes dimensional changes due to environmental differences, you need to keep moisture levels below 17%, continuously. In order to do this, any coating will need moisture vapor resistance in the 3% to 4% range. If more then this, the moisture content in the wood will eventually rise above the magic 17% figure and you'll have issues. In fact, 17% is pushing the envelop a bit. 15% is a better number and most formulators recommend less then this (12% is a common figure).

Shellac can't possably be expected to do this. Herreshoff Manufacturing built yachts were built to the highest standard, by industry leading experts, so naturally, surviving yachts fair quite well. They used a moderately heavy shellac coating between planking layers. Typically this would be a 3 pound coating, which is quite thick and it was used as a bedding compound, not so much as a glue or water proofing agent.

Simply put, to truly seal wood or plywood, you need to stabilize it's moisture content, which requires no more then 4% moisture vapor penetration. Generally, shellac will allow 12% to 15% which is way too much.

Epoxy is the real choice, which will lock vapor penetration down, below the magic number and stabilize moisture content. I recommend a 3 coat minimum to really seal plywood. It's usually sealed with 2 coats, but it's not the number of coats, but the film thickness that's important, so make it three to be sure.

Everdure and Norseal, will not seal the wood and you will have moisture vapor penetration, but they do come close, especially when coupled with a good primer and top coat. On a boat hull I couldn't recommend these types of products, but inside the hull it can have some benefit, except the bilge of course.