finishing question #1: polyurathane finish on stain

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Blog entry by Jeff Murdock posted 09-28-2011 04:45 AM 1236 reads 0 times favorited 5 comments Add to Favorites Watch
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I built a bookcase out of red oak. I used a Minwax stain after sanding. The stain dried for about 12 hours before I put Minwax fast drying polyurathane on it. About an hour later, I checked the finish and it was completely dry. I applied it in the shade so I decided to put it in the sun to allow the polyurathane to harden. 15 minutes later, I checked it and there were small bubbles all over the bookcase. What caused that?

-- Jeff, McDonough, GA

5 comments so far

View Fuzzy's profile


297 posts in 3406 days

#1 posted 09-28-2011 05:32 AM

Quite possibly it was air that was trapped deep in the pores of the oak. It expanded when it got hot .. the finish was softened a little by the sun .. the bubbles escaped the pores, but were unable to reach the surface. This is only my theory, but it is also the reason I start out finishing almost any project with several ultra thin(ned) coats of varnish … it seeps deeper into the pores and displaces the air so it’s not trapped in there. In this case, I don’t know if it would have made much of a difference because of the added heat from the sunlight.

-- - dabbling in sarcasm is foolish … if you’re not proficient at it, you end up looking stupid … ... ...

View iamMike's profile


8 posts in 2091 days

#2 posted 09-28-2011 05:35 AM

Did you cut the poly? if not do so. The bubbles are just how poly is thats why the recommend sanding between coats besides giving better bond for the next coat.

View Cosmicsniper's profile


2202 posts in 2576 days

#3 posted 09-28-2011 05:44 AM

Welcome to LJs, Jeff.

Most any finish (and even paint) can blister when exposed to hot weather (don’t ask me how I know this). This is because even after the poly (in your case) is “dry,” the curing process is still ongoing. When it dries too quickly, the evaporating solvent is being trapped under the rapidly hardened film and it has no choice but to form air pockets. Ideally, the surface hardens at a rate in proportion to the solvent evaporation…when it doesn’t, you are left with only one option…sand and refinish.

Oh, and just because it’s dry to the touch doesn’t mean it’s cured…solvent will continue to evaporate and oxygen will trigger the cross-linking process of the polymers in the finish.

Edit: seeing Michael’s post below, definitely try a chemical method if you see the blisters in time.

-- jay,

View Michael1's profile


403 posts in 2078 days

#4 posted 09-28-2011 05:53 AM

What happened was the sun cured the outer layer of the poly before the inside was dry. The first coat of poly will dry faster as allot of it is soaked up in the grain of the oak, however even though it was dry to the touch, the inside was still curing. When you placed it in the sun it cured the outer film of poly too quick and air from the pours of the wood tried to escape and as it did the poly was too set to level back out. Often times you here about production manufacturing curing finishes in an oven, which I do and have had this same problem if the casket is placed in the oven too soon. The oven curing is not to accelerate the drying process but the final curing so to not damage the finish in hand rubbing or in shipping.

The good news is your project finish is not lost, but what ever you do do not put sand paper to it right away as this will only clog the sand paper and damage your finish further. I would recommend taking a solvent that is equal in strength to what you thin your poly with. Typically when using poly i thin with Naptha, so I would try Naptha first and rub the finish down. If it has cured too long and needs a stronger solvent you can try something stronger. I am not sure if Xylene is stronger but you can try it. If it has cured for a few days than I would suggest a stripper to remove the poly before you do any sanding on it.
When you apply a finish again, I recommend reducing poly with a mix of 40% thinner, 60% poly. This is especially the case if you are applying it with a spray gun as it allows the poly to flow smoother. If brushing I would still thin it as it will accelerate the drying time and prevent any impurities from getting in the finish or since you are finishing outside, insects and small bugs always seem attracted to a freshly laid finish. The thinner allows the poly to level easier too.
As far as finishing outdoors, always keep your project in the shade. Even though there are some good Polyurethanes that are designed for exterior projects, Finishing in direct sun is not recommended as the UV rays will damage the sheen and break down your finish before it is fully cured, and some times cause air bubbles.
It is important to remember that even though the poly is dry, say 4 or 6 hours after it is applied, and in humid weather I have seen poly take 12 hours to dry, it is not fully cured for several days. Some finished state on the can that it will be fully cured in 10 days but most that I work with are closer to 21 days.

If after yo have tried the above methods, you still have trouble or questions, let me know. Chances are I have run into just about every kind of finish problem in the past.

-- Michael Mills, North Carolina,

View Cosmicsniper's profile


2202 posts in 2576 days

#5 posted 09-28-2011 05:58 AM

BTW, there is truth to what Fuzzy says about oak, however, I think it still depends on the finish itself. For example, this summer, I finished some red oak cabinets doors. I sprayed much of my color with a toner coat (dye mixed into wax-free shellac). The shellac was long ago dry and hard before finishing with some water-borne urethane (General Finishes Enduro-Var). Because I typically have to spray outdoors (I have a garage shop), I was left little choice but to fight the 105 to 110 degree temperatures. Even though the shellac didn’t bubble when I sprayed it, the urethane did. This lends credence to the view that it’s not just air in the pores causing the bubbles…likely it’s solvent still needing to evapoate yet being trapped beneath the hard outer layer of the finish.

Of course, I’d had this happen long ago with the same Minwax poly you used, however, it’s funny how we tend to forget about those things…or get a false sense of things after so much relative success. Shellac, my favorite product, has a way of doing that. Even if the shellac did bubble, the next coat would have just melted it again. Something like poly hardens in layers.

-- jay,

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