Bubbling Failure with Shellac and Spar Varnish on Garage Door

An unusual coating combination fails dramatically in an exterior exposure. December 30, 2007

I custom built an overhead garage door, made of cypress. After it had cured in the shop for two months I installed the door, and the next day, bubbles appeared - some as large as a quarter.

The finishing process goes as follows:
1 coat of Olympic watergaurd
1 coat of de-waxed shellac sealer
1 coat of Olympic gel stain
2 coats of Man of War spar varnish

What can be causing the bubbles to occur and what can I do to correct the problem?

Forum Responses
(Finishing Forum)
From contributor H:
I bet the sun was shining hard on the door? What's with the Waterguard and shellac? Shellac is not an exterior product.

Cypress... if it's dry, just stain and spar. Would you stain your house with gel stain? Why use it on your garage doors? They are not protected by a storm door or porch roof. I think the bubbles are from the Waterguard, shellac, and gel stain off-gassing under the dry, flexible, spar finish.

From contributor T:
A simple way of checking this is to take a needle and see what layers the bubbles go to. Whatever the sun is causing to gas, you should find it there. You cannot come up with a solution until you know exactly where the problem came from.

From contributor R:
Shellac is the culprit. Not a good coating for exterior applications. Don't have a solution to your problem now that the coating will not last long. Strip the spar off, remove the shellac, sand completely, and recoat with the spar alone.

From contributor D:
Shellac bubbles easily and quickly under heat and it does not matter how long it had cured prior to being subjected to heat.

From the original questioner:
The bubbles are the spar topcoat. I can scratch the bubbles, which are flat now, and the gel stain is still intact. I used this process on an oak entry door, without the Waterguard, and it's been holding up just fine for two years now. The Waterguard was recommended by a sales rep as pre-treatment. He said it would extend the life of the cypress. And the de-waxed shellac was used to give additional control of the gel stain.

From contributor R:
Did you thin the shellac to act as a wash coat to control the wood's absorption of the stain? I think if you placed the gel stain on top of full strength shellac, the gel stain really never penetrated the cypress, but just laid on top of the shellac. How did you apply the coats of spar varnish?