" " wisno wood furniture finishing: Bubble problem in wood furniture finishing

Monday, April 19, 2010

Bubble problem in wood furniture finishing

bubble in the wood finishing

Bubble and blistering in the wood finishing.

Bubble and blistering is one of the problems that often arise in the finishing process. Bubble is caused due to the trapped air bubbles within the coating film during the process of drying the wet paint and stuck at a dry film. If this bubble is sanded well and then coated with another clear coating, they will create pinholes or another bubble on the film layer. Bubble and pinhole is certainly disturbing and become big problem for the clean looked finishing. For some antique finishing, the bubble and pinhole could be camouflaged by some antique effects such as warm hole and spatter. 
 The cause’s of the bubble 
Bubble is one of finishing problem that is difficult to be handled. If already occurred bubbles or pinholes in the film layer, the solving process become very difficult and need a long time and  big effort. For some cases the stripping and wash off the finish and refinish is even easier to do than repair the bubble problem. Since that it is better to avoid the bubble problem than repair the it when it happen. To do that we need to understand the thing that cause that problem, and try to avoid it.  

There are some things that can cause bubble. 
  • There is residue of bleaching chemicals in the surface.
Bleaching is a process to lighten the wood color chemically. Mostly the process of bleaching is done with hydrogen peroxide. Hydrogen peroxide requires sufficient time to be complete his reaction perfectly. If the chemical is still there in the wood surface when we apply coat on it, the existence of residual peroxide in the wood will keep react and it will cause bubble to the clear coating layer. Therefore if we bleach the wood, we need to make sure that the bleaching process is really cured (the chemical reaction is finished) before start the finishing process. From my experience it takes at least 24 hours to ensure that the bleaching reaction has been completed and the surface was safe enough to be followed with the finishing process.
  •  The applications clear coating that is too thick.
A layer of clear coating is always need some time to release his thinner either with or without chemical reaction therein to achieve the dryness. When a wet coat is too thick, then the releasing of his thinner will be more difficult and take longer time since thinner should through the thick films. This will greatly increase the risk of bubble. Therefore, to facilitate the drying process to be faster and easier happened we should avoid too thick application. The proper application of clear coating will reduce the risk of bubble occurrence in film layer. Do the clear coating applications with normal viscosity. The viscosity should according with the spray system (about 12-15 second with the N.K. # 2 cup). The better film will be obtained with 1 layer wet coat, dry and sanded  followed with the next coat than the application of 2 layer or more wet coat without sanding between it. Two coat application with one sanding operation  still can be done, but after the first coat application we need to wait for awhile to let  the first coat is dry little bit, before do the second coat layer.

3. The glaze that is not dry 
Sometimes we use glaze to produce the finishing look we want. We need to understand that the glaze is a slow dry finishing material. The glaze requires longer drying time then the other stain. The glaze that is not dry will leave the residual solvent that can resulting bubble in the clear coating layer thereon. Therefore make sure that the glaze is completely dry before apply clear coat on it.  

4. Inappropriate filler application.
Filler is material used to fill the pores and grain of wood. There are some types of wood that has unique pores that is the small but deep. The wood such as mahogany, okumee, nyatoh, teak are some sample of the types of wood that is quite difficult to be filled. If the filler is too thick it will be difficult to enter and fill inside the pores. While the imperfect filling process will  higher the risk of  getting bubble. Therefore do the filler applications carefully especially for the difficult  type wood, do a little dilution of the filler to ensure that the filler has completely come in and fill the pores. Some time we need to fill 2 times to get the best result. Make sure also that the filler is fully dry before continue with the next finishing processes.  The residual filler solvent will effect like glaze  solvent in the finishing.

5. The air humidity and temperature.
 
The other factor involved with the bubble problem is the environmental factor, the air temperature and humidity. The high temperatures and low humidity will make the evaporation process becomes faster. Too fast thinner evaporation will generate bubble in the film layer. Do check  and record the temperature and humidity in the finishing room every certain time, make record the condition when the bubbles are arisen. Based on this record so we can more careful in do the finishing process when the environment is too hot or the humidity is low. For some cases where the air is too dry or too hot temperature, we need to do a certain treatment to lower air temperature by wetting the floor of the finishing room. The wet floor with good air circulation will help to lower the room temperature.

Modern Coating and Drying Technology (Advances in Interfacial Engineering Series) 


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