This section is from the book "Cyclopedia Of Painting", by George D. Armstrong. Also available from Amazon: Cyclopedia of Painting.
Before applying the second or middle coat, be sure the priming coat is hard dry over the entire surface. As this is the medium between the foundation or priming coat and the protecting or finishing coat, extreme judgment must be used in mixing the paint for this coating. It must not be too elastic and should dry without a high gloss. The paint for this coat, being the easiest working of any applied to the building, requires thorough and careful brushing to assure satisfactory results. Reshel-lac knots or sappy places if necessary. Knife putty into cracks, seams or nail holes. The paint should be mixed heavy so as to brush out well, also assist in filling and penetrating the priming coat, leaving a surface to which the finishing coat will readily adhere, as well as a surface which properly dries from the bottom out.
Too heavy an oil reduction will leave a high glossy surface over which the finishing coat will not adhere or properly dry. The reduction should be with sufficient turpentine to form penetration and still make a paint which will be elastic enough to withstand contraction and expansion and dry firm. Over such a surface the finishing coat can be brushed out smoothly and evenly without crawling or slipping under the brush. The paint will dry without danger of puckering, leathering, or flattening of the finishing coat as would be the case in a short time if applied over a high gloss. It is also very apt to crack and peel if oily coats are applied one over another. It is almost impossible to have solid painting with an excess of oil in undercoats as the coats will most always be spongy, rarely adhering closely to one another.
See that the undercoat is hard dry over the entire surface. The surface should be perfectly clean and free from dust and dirt. Reputty where necessary. Follow the same precautions as previously given for finishing coats. Brush thoroughly and carefully. Use a full stock brush properly broken in. Do not use new brushes for finishing coats. The paint for this coat should be the most elastic one applied, as it must stand the most severe exposure. It should be of good consistency with a full oil reduction, mixed so as to brush out smoothly and evenly, remain where left without danger of running or sagging and dry from the bottom out. The drying and gloss are always assisted by having the under or middle coats properly reduced and applied. Follow previous instructions as to cleaning off body color on parts that are to be trimmed. Bring down and square up the work so as not to show laps or poor workmanship.
Be sure the surface is dry. Do not use tar oil or other offensive smelling oils that will ruin the cistern water. Turn supply pipe from cistern when painting the roof. Mix the full amount of paint required for the first coat, as it is very difficult to make two mixes for shingles which will appear the same. Apply uniform coats to prevent spotting. Have the priming coat thin so it can be easily worked into the cracks. Keep ladders from resting on tin or in gutters. Hook over the comb of the house. Trim the ridge-board and coping as the work progresses. In doing this work do not go over the roof with the ladders after it is finished. The life of a shingle roof can be more than trebled if the shingles are dipped into properly prepared paint before being laid.
In dipping the shingles, they should be dipped at least eleven inches. This will allow 4 1/2 inches to the weather and 6 1/2 inches for the under lap. Never dip damp shingles; break the band around the bunch and spread them out to allow of drying before dipping or applying the paint. For dipping shingles, use paint of the proper consistency for finishing coat, reduced with not less than 50 per cent raw linseed oil. When the shingles are laid, finish with one coat of paint of a finishing coat consistency. Remember the roof is subjected to very severe weather wear and soon shows defective work.
The paint for the roof should be of good material. A mistake which is often made is that a very cheap mixture will do for shingles. Have the priming coat thin and enough of it mixed at one time to cover the entire roof. Keep the paint uniform while working and avoid having heavy laps or spots, as they will soon show through the second coat and make an ugly looking job. The second coat should be of good consistency and be well brushed out, using care to keep from applying the paint unevenly.
 
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