Blotchy Problem in the Wood Finishing

Blotchy is uneven coloration in a finishing color. This has become a 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 dark-colored spots on the finishing and make the finishing look 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 colored in the wood to make the resulting color look 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, it will obviously seem. The wood with uneven porosity also becomes a problem since it will absorb the stain unevenly, consequently resulting in blotchy or uneven color.

Principally, there are three things that cause blotchy finish, i.e., uneven stain application, uneven wood surface absorption, and improper stain selection.

blotchy color 

• The wood surface. 

Stain 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 depends on the type of wood and the roughness of the 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, he cannot cut anymore but burnish or polish the surface. Polishing cannot cut the wood surface and remove the wood fibers, but makes the fibers just lie on the surface; consequently, the resulting surface will have uneven porosity. When stain is applied to the wood, it will produce an uneven coloration (blotchy).

Therefore, the soft wood requires slightly different sanding techniques 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 well sanded also should be immediately followed with the finishing process. Wood is a hydrophobic 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 a long time usually needs to be resanded before being sent 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 a lot of exercise and adequate time to master a good spray technique. The improper spraying technique can be caused by the people not having 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 over the results obtained. The staining results will be highly dependent on 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 a good finishing result. Glue sizing is another technique that can help the preparation of the soft wood.
 

• The selection of stain

The stain character is one other thing that is a very important factor in determining the color results. Stain with quick drying thinner will make it dry fast, making the stain 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 be more wet and penetrate into the surface and be more absorbed by the wood. Then it will have a higher risk of 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 that is too quick to dry will give a streak line 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 takes a 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.