Outside work should always be washed down prior to painting.
Be especially alert not to miss knotting any of the small knots in light tinted work ; brown spots will come through in a few weeks where the knots are missed.
Rubbing Down. In rubbing down, if three pieces of pumice are used alternately, and indiscriminately rubbed face to face occasionally, they will produce an absolutely level surface.
Tar Spots. Upon outside work it sometimes happens that tar spots are found. Tar is very destructive of paint, and they should be thoroughly washed off with turpentine. Brunswick or Berlin black are also dangerous if painted over, their composition being bituminous.
Painting Bound Edges. In painting the wood- work of an ordinary room, paint well round the edges so as to paint under the edges of the wall paper, and well stop the angles formed by the wall and the architrave mouldings, and other wood-work. This keeps out dust and causes the edges of the paper to adhere thoroughly.
Dusting. The duster must be in constant use, as the con- tinual moving and dusting creates fresh dust continually, and all dust adds to the roughness of the work. Fat Edges. Fat edges must be always guarded against. This common fault is that of allowing the paint to accumulate on the edge at right angles to that which is being painted. The brush should always be drawn out toward the edges of the work, and not in from the edge, and any accidental accumulations must be lightly removed with the point of the brush.
Hints on flatting. If flatting does not turn out solid and satisfactory, the work must be repainted in oil colour. It is quite useless to attempt to reflat on a flatted surface, as the new coat will dissolve the former one and cause it to work up, making a worse finish than before. The rougher the wall the stouter it is desirable to use the flatting colour ; and the rougher the stipple, the less the wall will show up any imperfections.
Flatting will do a great deal to hide unevenness and bad places in an old wall. Faults in Painting ;
Cracking. Cracking in paint is caused by the under coats of paint being more elastic than the upper one ; consequently, when they are expanded by the sun’s heat or other causes, the upper coat is not accommodating and splits. The same result is brought about whether the elasticity of the under coats is due to their ingredients, proportions, or to their not having properly dried before the upper coats were put over them. It must be borne in mind that oil expands in the process of oxidising viz., the oxygen is added to it, and nothing is given up to make room for it. Turpentine, on the contrary, contracts strongly, especially if barely spread.
This can be well seen if a small patch of the two oils are put upon a piece of glass : when dry the linseed oil will show a wrinkled surface due to expansion, and the oil of turpentine will have a concave surface, and appear to be drawn in from the edges. If the superincumbent surfaces are not nearly related to each other in drying power, or if the varying power is not maintained in equal ratio, either cracking or blistering is pretty sure to result.
Blistering. Blistering is a more general fault than any other, and may well be termed the bete noir of the painter. It is brought about by various circumstances and conditions, but the actual and direct cause is always the same. Moisture is imprisoned, expanded by heat or other causes, and finds its neces- sarily enlarged accommodation in a blister, which will occur wherever there is least resistance, and where there is imperfect cohesion between the paint and its ground.
The moisture may be water, gas, spirit, or oil. It may be inherent moisture in the wood ; acquired moisture between the coats of paint ; resinous moisture from knots ; unoxidised oil in the paint ; water in the pigment, in the oil or in both, or a number of less usual faults.
Sometimes the work may be damp or frosty at the time of painting, and this dampness is shut in by the paint. Sometimes the wood itself contains constitutional water. Frequently there exists free resin oil in the knots. In any case the result is mechanically the same, the heat playing upon the surface ex- pands the moisture; steam or gaseous vapour is formed and the paint rises. A close examination of the blister will clearly show between which coats the imperfect adhesion allowed the blister to form.
Knots are frequently the locale of blisters, because inherent or acquired moisture in the wood itself naturally finds its exit through the open ends of the sap channels surrounding the knot. For a similar reason, cross-grained wood, because of the number of open sap vessels it contains upon its surface, will blister more than straight-grained wood.
The resin in the knot often gives rise to blisters immediately above the knot itself, because the resin oil keeps the colour soft or softens it and allows it to be expanded. Curiously, too, the very precautions taken to protect the paint from the action of the knot results in a smooth, keyless surface from which the paint is easily lifted by the vapour. This hard shellac in spirits has no affinity for the paint, and refuses to attach itself to it or to hold it.
If a blister be pricked when hot and rising, the pin-hole will allow the steam to escape, and it will not get any larger ; indeed, it may be pressed back into its place. All woods which show a large percentage of water in their analysis will blister readily.
To prevent blistering, care must be taken that due cohesion and relative expansion is obtained between the various coats of paint used, and that the particulars referred to as important in outside painting are attended to.
All knots, especially resinous ones, must be effectually treated, even to the extent of having very bad ones cut out and the places filled in with sound wood. If work is very much exposed to strong sun it is advisable to abstain from the use of a large proportion of oil, and to substitute an oil varnish for a portion of the usual oil.
The use of poorly- bound turpentine colour is not a cure, such colour having no protective power. A blistering tendency may be much aggravated by the use of ” fat ” colour. New oil colour should be used. Stale fat colour will blister of its own defects. Blisters are fairly sure to rise if the second coat is put on while the one before it is not thoroughly dry. The use of an excessive quantity of driers also leads to blistering.
The desideratum required both for preservative purposes and to prevent blistering is a perfectly homogeneous steam-tight jacket of paint, firmly attached to its ground in every part. If too elastic, it will blister on the slightest provocation ; and if too inelastic, it will crack. Paint will blister upon other paint, independently of the material painted, if the necessary conditions for a blister viz., imprisoned moisture and imperfect adhesion are present, but this is, of course, much less frequently the case upon other than wood surfaces. So-called blisters upon cement and stone are frequently caused by the action of nibs of unslaked lime ; those upon iron are caused by rusting in spots.