Bubbles and Blisters in Topcoat

[Robert Nease]

Hi,
I recently finished two outdoor barbecue tables for big green eggs, using edge glued eastern cedar. Sprayed a few coats of a 1lb cut of shellac flakes, allowed to dry, sanded to 320, cleaned, and then sprayed several coats of general finishes exterior 450 clear. The top coat bubbled up and "blistered" after it was dry and outside. I'll have to scrape and respray, but was wondering what the cause of this bubbling might be. Any ideas?:mad:

[Brad Wood]

i'm pretty sure you need to use dewaxed shellac if you are using shellac as a barrier coat between whatever and your topcoat

[Robert Nease]

Thanks, Brad. I AM using dewaxed shellac.

[Brad Wood]

ahh, well, you didn't specify so I made the leap.

in that case, I'm not sure.

I've learned the hard way that Target Coatings EM6000 doesn't apply well to heavily applied shellace (dewaxed), but other than that, I guess I can't help... so I guess I keep an eye on other replies :)

[Prashun Patel]

How hot was it outside? How many coats of shellac? How long did you let it dry?

The hot sun will do cruel things to some 'dry' finishes. Jeff Jewitt explained to me that successive shellac coats too close to each other can result in trapped solvent. If given enough time, at moderate temps, this alcohol will make it's way out peacefully and slowly. If you heat it, though, you volatalize a larger volume quicker. This can cause blistering or bubbling.

He nor I am CERTAIN this is what happened to my piece, but it sounds plausible.

I try to always let things dry out of the hot sun now...

[Bill Rogers]

How dry was your wood? I recently had a similar problem with a table top that I finished with Pratt and Lambert's 38 varnish. From what I read it was most likely caused by the varnish trapping moisture that was still in the wood.

Bill

[Robert Nease]

Thanks to you all. I believe that Prashun has probably got it right. I allowed the shellac to dry for a few days, but I would be willing to bet that when I moved it to the sun, the heat released some additional solvent, causing the quarter sized blisters. Live and learn, huh?

[Prashun Patel]

Thanks to you all. I believe that Prashun has probably got it right. I allowed the shellac to dry for a few days, but I would be willing to bet that when I moved it to the sun, the heat released some additional solvent, causing the quarter sized blisters. Live and learn, huh?

Couple days? Mine did it because I did a few shellac coats in the same day.

[Leo Graywacz]

Shellac should not be used outside.

[Neal Clayton]

the sun will destroy shellac in a matter of minutes. leo is right.

if you leave flakes outside long enough to go get a cup of coffee, come back and you'll have a puddle ;).

the only remotely outside use for shellac that i've ever heard of is a sealer underneath glazing putty since it's an oil barrier. but anything that ever even sees a little bit of direct sun cannot have exposed shellac.

[Chris Padilla]

Frankly, I'd leave the cedar unfinished BUT, you can use this stuff like I did for my cedar gate:

http://www.nam.sikkens.com/product-category.cfm?product_category=exterior

Sikkens makes some good stuff. My gate looks good still but it won't go through the turmoil a cedar bench surrounding BBQ grills will go through. Cedar is soft...very soft...it will ding if you look at it funny.

[Robert Nease]

There may be some confusion surrounding my question - First, the shellac was mixed from flakes of blond dewaxed shellac, which was not left outside or mixed outside or sprayed outside. Second, it was applied merely as a sealcoat for the top coat, and this was done inside, and allowed to dry before sanding.
Third, the top coats were also allowed to dry, and very lightly sanded with 320 before the next coat was applied. I like the look of raw cedar, too, but am pretty messy when it comes to grilling - so, I was shooting for a surface that I can wipe with a damp cloth to remove spills, charcoal dust, etc. It really turned out well, other than the blistering effect, which was not over the entire top, but just quarter sized spots here and there. It won't take but a few minutes to sand off the topcoat and reapply, and my thought is that it will "take" this time without blistering. I was just wondering if anyone else on the creek had a similar experience, and knew what had happened. I'll post a pic, once I have redone the finish. Thanks to you all, I am constantly learning here, and appreciate the time and thought that went into your responses.

[Leo Graywacz]

If it is outside shellac cannot be used.

Even if it is covered by other coatings.

It doesn't have the elasticity that a coating requires for exterior use.

[Chris Padilla]

Robert,

We may not be answering your question directly but using a FILM finish on an outdoor project is a lesson in futility. The wood is going to move a lot being outdoors and it will crack any film finish because they simply aren't flexible enough to move with the wood.* That is why most outdoor finishes tend to be of the oil variety and tend to need replenished once a year. Shellac will never make it. That is why I posted the Sikkens product.

Next you are using a VERY SOFT wood as a desk/bench around a hot grill. Believe me when I tell you how soft cedar is! I built a large gate out of it and had to be REAL careful moving boards around or even sliding one board on top of the other. It would just mar and ding so easily. Just an FYI. Cedar is great outdoors because it is fairly rot resistant and it does work nicely as a gate but I'd think twice about using it as a bench.

* - I think a Spar Varnish is an exterior film finish...it is flexible.

[Robert Nease]

All, Good conversation and thoughts.
Chris, you are totally correct, as are all others stating not to use shellac outside - period. General finishes confirmed as much, so I am sanding it off and just applying the exterior water based product - exterior 450. It's not a bench, though, it's just a tabletop, and won't be subject to much of anything except plates of uncooked and then cooked food. You're right about it being soft, but that's perfectly fine with me. I like the contrast of the SYP base and the cedar top, and fully expect characteristic dings and dents, as well as some mild checks. I have to tell you, I REALLY like SMC. If you are utilizing it, and are not a contributor, please consider being one. It is user supported, and is an invaluable resource for dialog and information - such as not using Shellac outside.
Thank you all again.