This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
In preparing a violin for varnishing, commence by sandpapering it all over with No. 1 paper and freeing it from scratches. Go over the entire surface lightly with a clean, slightly damp sponge, and when the wood is dry it will be quite rough again; rub with No. 0 paper till smooth, and repeat the damping and papering until a dead smooth surface is obtained, quite free from scratches. It is not usual to stain violins, as a much finer effect is got by incorporating the colour with the varnish, The following process will give excellent results. Dilute 4 parts of good copal varnish with 1 part (by measure) of turpentine, and heat it quite hot, being careful not to let it catch lire. Go over the entire violin with this with a stiff brush, and rub in as much as it will take at one coat; this will not be much if the wood was well finished. When it is quite filled, make a pad of cotton-wool, done up in a fine cotton or linen rag, moisten this with turpentine, and clean the surfaces of the violin as rapidly as possible; then put on a coat of spirit varnish, made thus: Colour \ pt. of methylated spirit with turmeric and red sanders wood.
In another 1/2pt. of methylated spirit dissolve 2 oz. of gum sandarach (juniper gum). Mix the two together, add two table-spoonfuls of Venice turpentine and 2 oz. of white shellac, and when dissolved, filter through cotton-wool or tine muslin. This elastic spirit varnish gives the violin the warm amber colour so much sought for. Lay on the varnish carefully with a large, round, camel-hair brush, avoiding streaks, and not going twice over the same place. It will dry very quickly, and three or four coats may be put on daily till the desired colour is reached; rub down with finely sifted pumice-powder and water and a woollen rag after every third coat. When a good body of varnish is on, the surface must be rubbed down with the pumice-powder till it is dull and smooth all over; the pumice is then thoroughly washed of. The final polish is obtained with tripoli and water, or crocus and linseed oil, on a rag, as before. After this is cleaned off, a brisk rub with the heel of the hand will give a surface like glass. The above instructions are applicable also to re-varnishing an old violin; but then it is neeessarv, in the preliminary sandpapering process, entirely to remove all traces of the old varnish.
When that has been done, the work is identical with the above.
Both oil and spirit varnishes are used on violins; the former give quicker results. Oil varnishes should be allowed an interval of at least two days between each coat; each kind of varnish should be dulled with pumice before applying another coat. Coating with boiled oil before varnishing is not advised. A yellow tinge may be imparted by the aid of gamboge and turpentine. A quantity of essential oil of turpentine being put in a cup, it should be placed in a water bath on a gas or oil stove and brought to a gentle heat and as much gamboge added as the oil will take up. Carefully strain, and apply with a camel-hair brush; a second coat may be given in three hours' time. The first coat of good spirit varnish may be applied the next day.
 
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