The composition of paints should be governed by the nature of the material to be painted. Thus the paints respectively best adapted for protecting wood and iron differ considerably. The hind of surface to be covered, i.e. a porous surface, requires more oil than one that is impervious. The nature and appearance of the work to be done. Delicate tints require colourless oil; a flatted surface must be painted without oil, which gives gloss to a shining surface. Again, paint used for surfaces to be varnished must contain a minimum of oil (see p. 433). The climate, and the degree of exposure to which the work will be subjected; thus for outside work boiled oil is used, because it weathers better than raw oil. Turps is avoided as much as possible, because it evaporates and does not last; if, however, the work is to be exposed to the sun, turps are necessary to prevent the paint from blistering. The skill of the painter also affects the composition; a good workman can lay on even coats with a smaller quantity of oil and turps than a man who is unskilful; extra turps, especially, are often added to save labour. The quality of the materials makes an important difference in the proportions used. Thus more oil and turps will combine with pure, than with impure white lead; thick oil must be used in greater quantity than thin oil. When paint is purchased ready ground in oil, a soft paste will require less turps and oil for thinning than a thick paste. Lastly, the different coats of paint vary in their composition : the first coat laid on to new work requires a good deal of oil to soak into the material; on old work the first coat requires turps to make it adhere; the intermediate coats contain a proportion of turps to make them work smoothly, and to the final coats the colouring pigment is added, the remainder of the ingredients being varied as already described, according as the surface is to be glossy or flatted.