Home>Discussions>PAINTING & FINISHING>Urethane over Shellac? Shellac over Urethane? Shellac outside?
7 posts / 0 new
Last post
DeborahB
Urethane over Shellac? Shellac over Urethane? Shellac outside?

It seems you cannot use urethane over shellac, because it will not adhere.

But what about shellac over urethane? Shellac adheres to almost everything, it seems. (It's too much of a hassle to remove the urethane at this point, but we want a shellac finish).

And is anyone aware of contraction problems and cracking with shellac if used outdoors?

Many thanks!

Faron
Re: Urethane over Shellac? Shellac over Urethane? Shellac outside?

Sorry!
Shellac isn't an "outdoor tough" resin.
It's OK for EXTERIOR spot priming (as in Zinsser's BIN primer, which is shellac-based), but it's just too brittle for outdoor use.

SPAR Polyurethanes are much tougher, & more flexible for outside use.

You'll have to evenly scuff-sand your existing poly, and apply further maintenance-coats of the same.
Even these SPAR-Urethanes require maintenance-coats to stay ahead of harsh sun & moisture damage.
Shellac just doesn't stand a chance...

Faron

ordjen
Re: Urethane over Shellac? Shellac over Urethane? Shellac outside?

Deborah,

De-waxed shellac can be used under urethanes. Such shellac is often labeled as a "universal" sealer. Zinsser sells one version.

One of the reasons shellac is not suitable for exterior use is that it is a terrific vapor barrier. Unfortunately that is not a good quality in an exterior coating. Exterior coatings should be able to shed flowing water, yet be able to allow vapor to pass on through. A coating that cannot breathe will peel. For example, the reason acrylic paints are generally better as exterior house paint is that they allow vapor to pass through them much better than oil paints. When a house's paint cannot breathe, it peels.

I generally don't like varnishes for exterior use either. They look beautiful initially, but are a constant maintenance issue.
Varnishes are also vapor barriers. My rule of thumb is don't varnish items where you can't keep the water from working its way into the wood.If it gets in, it too will cause peeling. I would prefer oiling exterior furniture. It may take yearly oiling to keep it looking good, but you will never have to strip it when it peels. Similarly, I don't like varnishes on garage doors. In the long run it does not hold up do to the stresses placed on the panels while going up and down. Many door manufacturers state their warrantee is void if you put urethane varnish on their doors. Spar urethanes are marginally better in this application as the film is designed to be more flexible and stretch with the flexing of the door. However, on my own redwood garage doors, I prefer to give a yearly coating of oil. It only takes a few minutes and it will never peel.

DeborahB
Re: Urethane over Shellac? Shellac over Urethane? Shellac outside?

Thank you Faron for laying the shellac hope to rest; I just love the finish, especially the amber color, it's a shame there is no approximation for exterior use (the dewaxed seems to go on clear and has no tint).

Spar urethane it is, and it does not look bad at all; quite well taken care of.

And, Ordjen, that is very interesting. I always assumed oil paints are better for outside, because more durable, but this explains why you see all this peeling even a couple of years after an exterior paint job!

On this, do you have any opinion about the Water-Based Exterior Spar Urethanes? Varathane makes one, though other brands don't. Might this address the problem?

I tried it on a test area, it goes milky when applied but then dries to a nice neutral varnish. Since it's water based, might that allow vapors to circulate out of the wood and ensure drying?

Again, what a great collection of experts!

J Roper
Re: Urethane over Shellac? Shellac over Urethane? Shellac outside?

It's accurate that shellac cannot be used outdoors. It is just not tough enough. The orange color that is a popular look of shellac can be achieved by tinting spar-eurathane. Any clear finish has limited life when exposed to the UV rays of the sun.
Also you're right that shellac over 'eurathane is okay but not 'eurathane over shellac.

ordjen
Re: Urethane over Shellac? Shellac over Urethane? Shellac outside?

Deborah,

One possible advantage of the water based urethanes is that they dry crystal clear. Oil urethanes always have a slight amber tone.Whether or not they are more durable, I am not sure. My biase is toward the oil based urethanes though. You never mentioned exactly what you want to varnish. As stated, if I were finishing something like an adirondack chair, I would not varnish it as all the jointery assures that water will eventually enter the wood and peeling will follow. Similarly, to varnish a deck would also be folly. It would look great the first year and commence peeling the following year. Again all the joints, surface nailing/screwing assure that moisture will get into the wood. When the sun shines on the wood, the heat generated forms vapor pressure and the varnish film pops. If you can eliminate either sun or water or preferably both, varnish would last, but the real outside world usually has both.

I live in Western Oregon and given my choice between building a wooden deck or a concrete patio on my new house, I chose concrete. I have refinshed far too many decks over the years to want to do it as a grow old.

Nick
Re: Urethane over Shellac? Shellac over Urethane? Shellac...

Wondering what exterior forces are the key concern when it comes to Shellac on an exterior door.

Reading here that the main threats may be moisture and sun, but is temperaturate also a concern/risk? The "exterior" door that I hope to use it on is very well sheltered from wind, rain and sun, but will see Wisconsin temps from -10-100F. Will it still be trouble? Appreciate ideas and answers. Live in a historically significant home and am hoping to use period-appropriate materials. We know that the original front door was treated with amber shellac.

Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.