Polyurethane Bubble Trouble

A milky cast and bubbles in a poly finish probably mean the catalyst is old, and there's too much moisture. October 25, 2006

Question
I have used polyurethane to coat a brittle compound to prevent it from breaking. I also wanted the material to be uniformly encapsulated in polyurethane without any bubbles inside. It is getting almost impossible to avoid bubbles. The bubbles keep forming after coating. Is this problem related to the usage of the hardener? Is there any other way to reduce the bubbles? The mixing ratio of resin and hardener is 10:4 respectively. The
solution turns milky white as soon as I mix the resin and hardener. Does this color mean anything? The brittle material is the size of a small ring.

Forum Responses
(Finishing Forum)
From contributor R:
The finish turning milky is often the result of an expired catalyst. Bubbles in the finish could be a result of the resin being exposed to excessive moisture - in this situation, the chemical reaction gives off carbon dioxide gas. It could also be from excessive wet film thickness.

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From contributor D:
I agree that your poly catalyst has likely expired. Polyurethane catalyst is hydroscopic, meaning it absorbs moisture, which makes it go bad. Make sure you keep it in a tightly closed container. Divide up larger amounts into smaller containers that you can fill to the top to keep out as much air as possible. You can also use a squirt of nitrogen gas in the can before sealing it to keep the catalyst fresh and away from exposure to the air in the can. (Paint manufacturers seal their paint can like this). Even after all this effort, the shelf-life of poly catalyst is rather short after being opened. Use it as quickly as you can, because its not cheap! A product called Bloxygen may help your catalyst last longer on the shelf.