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Decorating Defects - their cause and cure

Blooming of Varnishes and Enamels.

Blooming is one of the most frequent and troublesome defects encountered by the decorator in the course of his work and one which is all the more annoying because it may take place in spite of the most careful workmanship.
It shows itself in the form of a dull cloudiness on a gloss varnish or enamel film and may develop soon after application, or weeks or months later. If the surface thus affected is examined under a powerful magnifying glass, it will be seen that the cloudiness consists either of a kind of fine crystalline deposit or, alternatively, of microscopic pits or craters in the film; crystalline bloom can usually be removed without much difficulty by rubbing with a leather, but crater bloom withstands this treatment.
The cause of blooming is even now not completely understood; the trouble is not confined to any particular kind of varnish or enamel and may manifest itself on high-grade finishes as well as on those of inferior quality.
On occasion, it may originate from some chemical or physical reaction in the process of manufacture, but as a general rule it is due not so much to the material as to the conditions to which the latter is exposed during or after application.
These conditions include draughts and sudden falls in temperature, bad atmosphere due to poor ventilation, fumes from gas fires, proximity to industrial plants, and dampness in all its various forms. The most fruitful source of the defect is the deposition of moisture on the varnish film soon after the latter has been put on. Falls in temperature come under this category since warm air holds more moisture than cold air and the lowering of the temperature is accompanied by the deposition of condensed moisture, usually in so minute a form as to be invisible to the naked eye. For this reason, varnishing or enamelling in the late afternoon should be avoided, especially in the early spring or late autumn, when the evenings are, as a rule, appreciably colder, and for this reason, too, all possible precautions should be taken against draughts.
Blooming may also be caused by the presence of moisture on the ground work to which the finish is applied; consequently it is important to see that, after washing down has been done, the surface is perfectly dry before any fresh varnish or enamel is put on. It should be remembered that old paint or varnish films are often to some extent porous and are thus able to absorb and retain more moisture than is commonly suspected.
If strong solutions of soda or sugar soap are employed for cleansing purposes, or in the preparation of old work, careful rinsing must be carried out, for any traces of alkaline matter left on the surface will be hygroscopic, i.e. will have the property of attracting moisture from the air.
The defect is fairly common on varnish finishes on woodwork, or again, on wallpaper which has been varnished, and when it occurs in such instances it can often be traced to the fact that the size coating applied under the varnish is not perfectly dry when the latter is put on. Particular care must be taken to guard against this when paper is being hung and varnished over an old painted ground, previously varnished paper, or similar impervious surface. Both the new paper and the size coating take longer to dry in these circumstances than when bare plaster is being decorated and there is therefore more risk of the varnish being put on before all the moisture has evaporated.
The danger of blooming taking place seems to be greatest during the initial drying period of the finish, though the trouble may, and often does, occur after the surface is quite dry and hard. The use of a relatively quick­drying varnish, such as one of the synthetic-resin type, probably involves less likelihood of the defect.
Whatever the nature of the finish, however, it is desirable that while it dries, the film should not be subjected to injurious atmospheric conditions; these are most apt to be found in bathrooms and . kitchens where there is usually a good deal of condensation if the rooms in question are being put to their ordinary uses. Whenever possible, they should remain out of service for at least three or four days. Cases of crystalline bloom can generally be cured by rubbing with a clean dry leather, or, if this proves ineffective, by the use of a pad slightly moistened with linseed or olive oil. Crater bloom, however, can seldom be eradicated and it will be necessary to flat down and refinish. Broadly speaking, the thicker the film, the more prone it will be to the defect, so that the risk of blooming occurring can be reduced by spreading the finish as thinly and evenly as possible.