" " wisno wood furniture finishing: NC sealer

Wednesday, July 31, 2013

NC sealer


NC sealer (Nitrocellulose sealer) is type of sealer that widely used in wood finishing. This sealer is made from nitrocellulose resin as the main ingredient. NC sealer is liked by many people because it is relatively ease to be handled and applied. It is one component and gives the clear looked finishing film. As a sealer it serves to provide a film layer before top coat application. It is fast dry and easy to be sanded and giving the smooth and even surface before top coat application. It contains the zinc stearate powder as the sanding agent that will reduce the heat generated by friction between the coating and the abrasive in the sanding process.
Usually the NC sealer is applied by spray gun. The other equipments that can be used are curtain coater and brush. Application by spray gun will produce the perfect layer; smooth and even coating layer. Curtain coater coating could produce a flat and smooth surface with better efficiency, but it is limited on the flat forms. The applications with brush can be done with modifications in the chemical to minimize the brush marks. But some finishing peoples use the regular nc sealer to create the brush mark to make special finish.
NC sealer normally always needs to be sanded and coated with the top coat. It will help to give the film built in the finishing process, while the top coat will give the smooth surface and make the gloss. But sometime people use the sealer as the final coat; especially when the simple finish is required. The sealer layer will give the high gloss finis film and rather rough surface. We can rub the sealer layer to lower down the sheen and make smooth and soft surface. 
Always check the condition of the sealer before apply this material. Read carefully the data and instructions of uses about this material. Check the solid content if necessary, normally the ready to use nc sealer has solid content about 17% to 20 %. Then stir the mixture until it is homogeneously mixed. The sealer has some particle the will settle at the bottom, make sure all the particle spread in the liquid suspension when it applied. Check the material physically. The NC sealer normally is looked as clear yellowish, viscous liquid. Measure the viscosity materials. The spray application will need the viscosity to be about 14 second (# 2 NK 2 cup) or about 20 sec ( Zahn cup # 2 cup). If necessary we can cut it with lacquer thinner to reduce the viscosity. 
The spray application is the most used to layer the NC sealer. It will produce the best coating layer, smooth surface with even layer. But we need the proper spraying technique to get the perfect result. Avoid spraying the sealer too thick; 1 wet coat or maximum 2 coat layer is recommended. Let the layer dry and then sand the sealer to give the smooth surface. We can repeat the sealer application after the previous layer is dry and properly sanded. 
The stearated sand paper with grade: # 240, # 280 or # 320 are the most used to sand the sealer. The sanding is important step to make the smooth and event surface. The first sealer application will wet the wood and stiffen wood fibers that will make the surface rough. The sanding operation works to cut the wood fibers and produce the smooth surface.
 The very thick layer of NC sealer should be avoided since it could affect the finishing quality. Too thick sealer could lead to the adhesion problem or crack at the finish film. The maximum thickness for the NC finish is about 4 mill dry film. 
NC sealer is usually applied after stain application to protect the stain and prepare the smooth surface before the top coat application. Make sure that the stain is completely dry before the sealer is applied. The solvent base stain that is applied by spray usually will be dry in few minutes, but the water based or oil based stain especially when applied by wiping or brushing usually needs longer time to dry. Make sure the stain is dry before coat it with the sealer, the solvent that is trapped in the sealer can interfere the finishing quality.
NC sealer sometime is applied directly to the wood. It used when we want the natural color, then the stain is not needed. NC sealer can also be thinned and used as a sealer sizing. The sealer is applied to the soft wood to help the sanding process. The sealer is applied to the wood to stiff the wood fibers and make the uniform porosity to the wood surface. With the sanding process, the more smooth and homogenous surface can be obtained. The sealer sizing is only needed when we deal with soft and porous wood and with good control. The sealer layer at the wood will interfere the stain absorption and affect the final finishing color and looked. Make sure we don’t apply too thick sealer. Normally the sealer with maximum 10 % solid content is used to do the sizing. 
NC sealer is also be able to be applied over the other type of coating layer. This is usually necessary if we revise the old finish layer. NC sealer can be applied on most kind of coating layer; Shellac, Varnish, PU, Ac or UV coating can be coated with NC sealer. The coating layer must be completely dry when the NC sealer is layered on it. The good sanding at the previous coating is needed to produce a smooth and flat surface. NC sealer coating will not be well blended as the full nc system, then the smooth surface is needed to produce.
NC sealer is a thermoplastic material, he will be dissolved again if it is wet with solvent. NC sealer should only be coated with NC top coat. The reaction dry top coat such as: AC, UV or PU coating will dissolve and damaging the NC sealer layer.

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