Blotchy problem in the wood finishing

Blotchy in the wood finishing

Blotchy is uneven coloration in a finishing color. This is become finishing problem since it will produce an uneven color appearance. In the finishing process the uneven coloration could be very thin and almost invisible and it won't be a problem, but severe blotchy can be up to the occurrence of a dark-colored spots on the finishing looks and make the finishing is looked bad. Blotchy is mainly caused by uneven absorption by the wood surface or an uneven stain application. The absorption problem  is a risk when we stain direct to the wood. The stain application directly on a wood can produce the natural coloration in the wood finishing. This method will  make the stain diffuse, mixed in and coloring in the wood to make the resulting color looked will be very natural. The color result is almost as the real wood color. But this system requires good surface preparation, because if there is any imperfectness in the surface will obviously seem. The wood with uneven porosity  also become a problem since it will absorb the stain unevenly, consequently resulting to blotchy or uneven color.
Principally there are three things that cause blotchy in the finishing i.e.: uneven stain application, uneven wood surface absorption and the improper stain selection.

 
The wood surface. 

Stain is applied to the wood surface will be absorbed by the wood. The absorption of wood stain and other finishing materials depends on the porosity of the wood surface. While the wood porosity is depend on the type of wood and the roughness of wood surface. Wood surface roughness is primarily the result of the sanding is done on the wood surface. For the hardwood such as oak, mahogany, teak, rosewood, etc., the sanding can be done with relative ease. But for the soft woods such as maple, pine, sengon, etc., the sanding process is a little  bit more difficult. The soft wood contains more wood fibers, and the wood fibers will be attached to the sandpaper in the sanding process. When sandpaper is filled with wooden fibers then he could not cut anymore, but burnish or polish the surface. Polishing cannot cut the wood surface and remove the wood fibers, but make the fibers just lying on the surface; consequently the resulting surface will have uneven porosity. When stain is applied to the wood, it will will produce an uneven coloration (blotchy).

Therefore, the soft wood requires slightly different sanding technique than the hardwood. The sanding for softwood requires more replacement of sandpaper, the sandpaper that is already filled with the wood fibers should be replaced immediately. The wood that is already good sanded also should be immediately followed with the finishing process. Wood is a hygroscopic substrate; he will absorb water from his environment when he is exposed. The soft wood will tend to absorb more and make the wood surfaces become rough. The good sanded wood that has stayed for long time usually needs to be re sanded before send to the finishing process. 

The stain application
 

Stain application is one of the things that will determine the color results. The uneven stain application will give uneven coloration. The spray process is the best way for stain application, we can control the thickness and select the part of surface we want to stain by setting the spray. But the spray application requires good spray technique to produce a good coloring result. People need lot of exercise and adequate time to master a good spray technique. The improper spraying technique can be caused by the people is not enough skill or he has tired already and it will make the bad finishing result. The other way to apply stain is by dip, brush or wipe. This application needs different techniques with the spray. This technique is relatively simple and easy to do, but we have limitation control to the results obtained. The staining results will be highly dependent to the surface porosity and concentration of pigment in the stain mix. The imperfect sanding and preparation and uneven surface will give an uneven coloration. Since that the proper sanding is a very important factor to make the good finishing result. Glue sizing is another technique that can help the preparation to the soft wood.
 
The selection of stain

The stain character is one other thing that is very important factors in determining the color results. Stain with quick drying thinner will make it dry fast and making the stain does not wet and penetrate to the wood surface. This will reduce the possibility of blotchy. While the slow evaporate stain like water base stain or oil stain will more wet and penetrate into the surface and be more absorbed by the wood. Then it will higher risk producing a blotchy color. The selection of stain can be more flexible if we apply stain by spray. The selection of stain is relatively limited when we apply stain by brush or wipe, since the stain which is too quick dry will give a streak line by this way.

To reduce the risk of blotchy then we need a proper selection of stain. For soft wood, then we should not use a stain that take long time to dry.  For this wood we can choose a toner stain, a stain with a little clear coating in the mix to prevent  too deep penetration of the stain which has  high risk to get blotchy.