A few years ago, green lumber was so seldom used in building that the home mechanic was never confronted with the problem of how to paint it. Today, however, a great deal of green lumber is being used, and the sap found on the surface of this wood presents a serious problem. The best method, if possible, is to allow the wood to go unpainted until it has aged and most of the sap and moisture evaporated out of it. Green wood can be given a coat of shellac which will keep the sap from bleeding through the paint, but the shellac will seal off the pores of the wood and prevent the paint from making a strong bond with the wood.
Before paint is applied to unpainted wood, the surface should be free of dirt and dust. Paint penetrates the tiny pores in the wood, and if they are filled with dirt and grease, the work will not be successful. Turpentine can be used to remove the grease. All rough spots in the wood should be sanded smooth, and knots and sappy spots should be coated with orange shellac to prevent the sap from discolouring the paint.
The first coat
The first coat of paint should be thinned with a good grade of turpentine. The amount of thinner used will depend somewhat on the condition of the wood. If the wood is well seasoned and extremely dry, the thinner can be linseed oil. The purpose of the first coat is to fill the pores and form a strong base for the following coats. By adding more turpentine, the drying of the paint is retarded, allowing it to permeate the pores. If the wood is dry and the pores are open, however, linseed oil can be used instead. Turpentine will dull the gloss of paint, while linseed oil will increase it.
Dip the brush into the paint so that about two inches of the bristles are covered. Remove the brush and wipe off the excess paint by drawing the brush across the edge of the container. By repeating this process several times, the end of the brush will be well filled with paint, with no surplus to run down the handle. A paint brush can carry only so much paint. If you dip the entire length of the bristles into the paint and try to move the brush to the surface, you are almost sure to lose a quantity of paint in the process.
Brush the first coat of paint into the wood vigorously, so that not only is the entire surface covered with an even coat but the paint is forced down into the wood pores by the brushing action. Do not brush the paint too much after it has set, as this constant rebrushing will give a rough surface.
Start painting at the highest point and, if possible, work across the surface rather than up and down. This will prevent any vertical joints in the painted surface if you are unable to finish the job in one day.
Use a wide brush wherever possible, but keep a small trim brush for corners where the wide brush cannot be used without bending the bristles.
After the first coat is dry, fill all cracks and nail holes in the wood with putty. This work is done after the first coat is on because unpainted wood will absorb the oils in the putty and cause it to dry out and crack. Use a good grade of putty, work it into the holes with a putty knife, and smooth off the surface.
The number of days required for paint to dry varies, but allow at least a week and longer if necessary.
The thinning requirements for the second coat of paint differ somewhat from those of the first, or priming coat. While the purpose of the first coat was to penetrate the wood, the second coat must make a tight bond with the first and present a hard and non-gloss surface for the final coat. The second coat should completely cover the first and must, therefore, be somewhat thicker.
The final coat can generally be applied without
thinning. It should contain plenty of oil and no turpentine, and it will
dry to a glossy finish.