Avoiding Small Bubbles in Polyester

Polyester finish performance is sensitive to temperature conditions, thinning proportions, and other factors. This thread offers troubleshooting ideas along with examples of successful strategies. May 18, 2010

We’re doing two polyester finishes. One is black and one is clear. On the black we seal the panels, sand the primer, and lay down the black polyester. When the black dries it is smooth and glossy. On the clear we have tried two different methods. We have put down a tack coat and then about four or five minutes later we lay down the clear. We have also tried spraying a light clear coat, letting it dry and sanding it like a sealer. Then we lay down the clear polyester. On both methods we get the same results - lots of little bubbles in the finish. The problem we have is we have to sand a lot of material to get out the bubbles.

The black is on MDF and the clear has been on oak, hickory, anigree and lyptus. We’re in Sacramento, CA. The weather is cold and wet so we spray then we put the panels in a 70-80 degree heated room. When we lay it down we put on a ton of product. Why does the clear bubble and the black lay flat?

Forum Responses
(Finishing Forum)
From contributor S:
Are you getting bubbles in the clear coat you are laying down as a sealer coat and then also in the top coat? How many mils are you laying down on the sealer coat? What is the temperature of the substrate panels before spraying and temperature of the room you are spraying in? Which polyester are you using?

From the original questioner:
Yes we are getting bubbles in both sealer and top coats. I don't know the exact mills. I know we have tried a couple of different thicknesses. I coat up to 3-4 coats on the sealer. The panels and the material have been room temp, I'd say between 50-60 degrees. We have tried in the heat room and not in the heat room for drying and we get the same effect. I don't know the brand. I'm not the painter I'm the cabinetmaker searching for answers for our painter.

From contributor S:
For starters your room temp is too cold. Most finishes are formulated for 77F and 50% RH. Polyesters like 68F and above but you can go down to maybe 60F, not 50F. Place your substrate panels and finishing materials in the warm room long enough that they reach the 68 plus temperature all the way through and try moving them into the spraying area just long enough to spray one or two at a time then move them back to the warm room. Always keep your substrates and finish materials within 10 degrees of each other. See if that helps.

From contributor W:
This sounds like it could well be an equipment issue. What type of spray equipment are you using to apply the material, and what are the settings (air pressure, fluid pressure, tip size, etc.)?

From contributor R:
Are you spraying an isolante sealer before the clear or are you using the clear as a sealer?

From the original questioner:
I'll try putting the material and panels in the heat box to raise the temp before spraying. The gun is unknown to me right now. I do know it's a HVLD of some brand. We are not using an isolante sealer (I don't know what that is). We are using the clear as a sealer. My question is still; how come the black lays flat and the clear bubbles?

From contributor O:
I don't know why the black lays flat and there are bubbles in the clear, but I would maybe try two different things to narrow it down to a specific portion of the work.

1. Prepare an MDF panel with the primer and spray the clear on it rather than the black.

2. Prepare an MDF panel and use the same schedule you are using on the various woods you are using.

Bubbles in both would lead me to think it was the clear. Bubbles on the second but not the first would direct me to the primer. Bubbles in neither would tell me it's something to do with the wood. I don't know what I'd think if you got bubbles on the first and not the second, but you get the idea.

From contributor Y:
Isolante is just a 2k urethane formulated to go under poly as a sealer. There are many isolante products out there. Isolante is Italian for "isolation" meaning it isolates the poly from contaminates in the wood and such things as bubbling etc. It is also known in the industry as a "barrier coat" creating a barrier between the substrate and the polyester. To me it sounds like you’re having a problem of the material setting too quickly or not being thinned out enough to allow the bubbles to surface and burst. Most of the polyesters I used I thinned at least 10% Duratec/seagraves/ guardsman/chemcraft sadolin/etc. Seagraves had the best isolation coating called "rosewood barrier coat." The isolante is also very good but not meant for oily or waxy woods in particular. If you use a barrier coat sand and thin your material (start at 10% and go from there).

From contributor R:
The MDF isn't bubbling because there are no pores to trap air into and it sounds like you are using a primer as your seal coat, two different systems. Your polyester mfg will have an isolante for your system. Get some and use it on your clear coatings. I thin the first coat of polyester more than subsequent coats also so it flows into the pores and prevents bubbles (do samples first). Also you could get a rep to come in and spray some samples with you in your shop and guide you.

From contributor K:
I spray a lot of polyester where I work. Polyester is a real fussy product. It all depends on what type of wood you are spraying. If you are spraying a tight grained wood like cherry or maple then it’s a really quick process. I hardly ever get bubbles or pin holes. Now let’s say your spraying some wenge or mahognany - then you may run into that problem. This is my finishing schedule. First I spray a barrier coat and let that flash off for 30 minutes. When its 100% dry to the touch I begin my polyester. I thin the poly down 40% with acetone. It takes many light/medium coats. I go spray and let it flash for ten minutes. You don’t want to put it on too quickly otherwise you will get pinhole or solvent trapped below another before it has time to flash off. After the 40% I spray the 20%. This is where I complete the grain filling. After a few 20% then it should be all grained filled. My booth temperature is at 75 degrees and I use a Sata jet B with a 2.0 needle nozzle aircap set up. The polyester process takes me around 2 1/2 hours on average. Have patients and you shouldn’t have any problems.

From contributor Y:
It's amazing how many different poly techniques there are out there. Here was my techniques and gun settings etc. Devilbiss color primer gun with a 3.5 fluid tip and needle. Material wide open, fan at 4" at 2" feet away from surface for polyester.

1.8 fluid tip and needle for barrier coats, fan at 8-10" at 10-12" " away from surface. Apply a 4-6 mil barrier, let dry 40-50 min. scuff with 180 paper, tac and blow off surface. Apply 20 mil let tac 30 min, apply 20 more, let tac, apply the final 20 mil let dry. Total time per piece two hours.

There is still sanding and polishing left to do the next day or sanding and 2k coats and polishing depending on customers specs. I think you'll find everybody's schedules to be different depending on what product lines they are using, how much they are manipulated, and how much they are thinned. There are no hard fast rules that I know of. I do know my way never created problems mentioned here or elsewhere with my products.