Bubbles in topcoat

      Causes of air bubbles in the first coat. May 22, 2002

Question
I am finishing some oak ply pieces with no stain, just topcoat. After spraying the first coat, there were little bubbles everywhere. This is not fisheye - they are actual bubbles on the surface. This also happened - only on one little spot - on some maple ply I was spraying. What's going on?

Forum Responses
This is mostly caused if your coat is too heavy and/or if you have a porous wood like oak, air gets trapped under the coating and needs to break through, forming a bubble. Or in closed grain woods such as maple, solvents get trapped when the coating is excessive and the film starts to "skin", thus trapping the solvent that needs to release, forming a bubble. In most cases, they pop and now are called pinholes. Remedy using a retarder or flow enhancer or backing off on your application. Example: one light coat, allow to tack, then apply your second coat.



Along with too thick of a coating, you might be using too fast of a lacquer thinner or a lacquer that needs to be reduced. The newer low VOC lacquer normally is sold with the claim that it can be sprayed right out of the can, but typically it needs to be thinned at least 15%. Thinner coats and slower reducers will solve the problem. Spraying outside on a windy day can cause this problem as well.


I have found this problem also when spraying around heat. Heat makes the air in the pores expand and it can't leave because you just sprayed a finish on it, so it makes a bubble. The other reasons are true too.


Plywood will always be worse than solids for bubbles. I think it has something to do with the glue but am not sure. Reducing and retarding your sealer and topcoat more than you would when spraying solids will help. A vinyl sealer will also serve as a barrier between whatever is in the plywood and the finish and help prevent further bubbling.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
The bubbles may come from the entrapped air inside the pores of the ply surface. This can be solved by wiping the ply surface with a solvent by round action. There can be another cause of bubbling problems - if you are using PU, have look at the hardener you are using, because a milky looking hardener is already emitting carbon dioxide which when sprayed will produce a bubbling problem.



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