Glue size in the wood finish

Glue size or glue sizing is a finishing techniques at the beginning of the finishing process to help producing the even and smooth surface for the soft wood. The wood surface contains lot of wood fibers. The fibers will produce a rough surface at the application of clear coat and it is need to be cut and cleaned in the finishing process to produce smooth and even finish. The wood fibers also could disturbing the stain application, since it could make the rough wood that will absorb more stain and make dark color. One of the purposes sanding the unfinished wood is to cut off the wood fibers. But some type of wood such as: sengon, pine, agathis, (the soft wood) are difficult to be sanded since its fibers are soft and elastic. The sanding process often makes the fibers lay down on the wood surface and attached to the surface without being cut. For this kind of wood we may need to do "glue sizing" to help producing the smooth surface. Glue sizing is an application of a thin glue (water based glue) at the the raw wood, before the last unfinished sanding. A thin glue is applied by spray wet or brushed at the unfinished wood that has been finally sanded ( sanded with # 180 sandpaper). Let the glue dry and raise and stiff the wood fibers. And then scuff sand the wood with # 180 or # 240 grade sandpaper to cut the wood fibers to produce the smooth wood surface.

Glue size will also help to overcome the problem of the uneven porosity of the wood surface. Sometime there are some certain parts that have a higher porosity (soft places) than the other part. This soft part is rather difficult to be detected in the unfinished wood sanding process. The soft part will absorb more finishing material and could create problem in the finishing problem. The soft parts will absorb more stain causing uneven coloration, or blotchy color in the finishing.

This problem can be minimized by the glue size. When the glue is applied, then the soft wood will absorb more glue than another part. Then the more balance and uniform porosity of wood surface will be obtained.

Sealer size
 
Sealer size (sealer sizing) is a process similar to glue size, but it is done by using a sealer, or wash coat. A diluted sealer applied to the wood surface, let it dry and followed by scuff sanding to cut the raise and stiffen wood fibers. This will produce the same effects with the glue size. But the sealer used for glue size should the same type with the type of clear coating that will be used for the next finishing processes. The incompatible clear coating in a finishing system may cause finishing problems. We also need to consider that sealer that is thinner base material will also resolve with thinner, since that the next stain application should be done carefully especially when we use thinner base stain. If the apply stain too wet, the thinner stain will wet and resolve the sealer and it can create even a more blotchy appearance. Avoid the stain application that makes the stain wet the surface like dip; wipe or brush. The spray application is the best way since we can better control the stain application. We can do wipe or brush the stain if the stain thinner is a solvent that not resolve the sealer.

The things that must be considered in sizing.

The glue size in general will help the raw wood sanding process and bind the wood or veneer surface becomes smoother. Glue sizing will reduce the possibility of blotchy on the soft wood. But nevertheless this will affect the absorption of the stain by the wood. Application of glue or sealer that is too thick will cover and even block the surface of the wood, consequently it interfere the absorption of stain, and then it can reduce the beauty of the wood grain in the finishing appearance. Since that we need to make sure that the glue size should not be too excessive. The glue size should be done on certain wood that really need it. The glue size is usually done for type of soft wood such as sengon, pine, maple, aghatis, etc., or for the soft wood part such as: the sap wood.
On the hard wood such as teak, oak or mahogany, the glue sizing is not really necessary, unless for the sap wood that has high porosity. With the proper techniques of sanding  (see our articles how to sand the wood) we can produce a smooth and flat surface relative ease for the hardwood such as teak, mahogany, oak, mende, etc. Application of glue or sealer on the wood surface before stain application will disturb the subsequent staining process.
Further, the application glue or sealer should not be too thick. The thick glue will block the absorption of stain and will disrupt the performance of finishing and reduce or even eliminate the existing beauty of the wood fiber. While the beauty and uniqueness of wood grain is the great value that should be enhanced in the wood finishing process. The thickness of the glue actually depends on the porosity of wood, but usually the solid content of the glue or sealer is around 7% or less.