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Decorating Defects - their cause and cure

Graining Defects

The defects which occur in grained work do not differ greatly from those in other forms of painting, though in a few cases they originate primarily from the processes used in graining. For the sake of convenience, the main troubles lIkely to be encountered are summarised under this heading.
Cracking of Grained Work.-This is probably the most frequent kind of defect and may be due to one or more of several causes. The ground coat, the graining colour, or the varnish may be to blame, and it is important first to ascertain to what depth the cracking extends, Le. whether it goes down to the ground coat, whether the fissures occur in the graining coat, or whether the varnish alone is affected.
Decorators who take care to observe the rules of coat sequence in plain.
painting are sometimes apt to overlook them when it comes to graining. The use of a varnish which is too short and brittle, over a soft and relatively elastic graining coat, is a common fault and will almost inevitably bring about crazing or cracking, especially if the work is exposed to the direct rays of the sun.
If only the varnish shows cracks, therefore, it is probably due to the fact that it is not sufficiently flexible; many exterior varnishes which give good service over ordinary paintwork, or over stains, are not so satisfactory for graining purposes; whenever possible, therefore, use a good, long-oil variety for finishing the work.
It is possible that varnish is rather more liable to crack over grained work; this may well be due to the fact that the film of graining colour varies in thickness, owing to the combing and figuring processes, with the result that the surface tension of the varnish is not so even as in ordinary painting; this is a contributory rather than a prime cause, but it is worth while bearing in mind.
If cracking affects the graining colour also, the trouble may be due to the use of too much oil in the latter, to following on with the varnish coat before the graining colour has had time to harden all through, or to an excess of driers. Too much oil will make and keep the colour unduly soft and delay its drying, particularly if such pigments as Vandyke brown or carbon black, which have a retarding effect on drying, have been employed.
The application of a coat of varnish, unless it is very flexible, on such a ground may induce cracking. Excess driers are a frequent cause of trouble because many grainers habitually use them in paste form, partly as a siccative and partly as a kind of megilp. If too great a quantity is added, the graining colour skin-dries, the lower portion remaining soft and, consequently, more liable to movement.
Cracking extends to the ground coat less frequently. When it does so, it is probably because the latter is too soft and oily in relation to the graining and varnish coats. Some grainers prefer a ground with a slight degree of gloss (induced by a higher proportion of oil) to work on, but this risk must be taken into consideration.
Failure of Grained Work to Dry.-This is another not infrequent defect which may be caused by the presence of grease on the ground coat or, alternatively, to the use of too much driers. It is worth while remembering that some of the pigments used for the graining colour have poor drying properties and under unfavourable atmospheric conditions-especially in interior work where the ventilation is poor-drying may take somewhat longer than would be the case in ordinary circumstances. The addition of gold size. to the graining colour is to be recommended only with caution but there are times when it may be advantageous.
Blooming on Grained Work.-Blooming of varnish over graining is more likely to take place when distemper graining has been carried out than when oil graining has been done. In many instances it is caused by the application of varnish before all the water in the graining colour has had time to evaporate, and this frequently happens when substances such as stale beer or sugar have been added to the colour for binding purposes, or again, if glycerine has been used to retard the set.
In oil graining, a common cause of varnish blooming is the use of cheap paste driers; these often contain appreciable quantities of water which, if trapped beneath the varnish film, may be responsible for the trouble.
In most cases when blooming takes place on grained work it is for similar reasons as those which obtain when ordinary varnish or enamel films are affected in this way.
Deadening of Varnish in Grained Work.-Occasionally it will be found that the varnish coat on grained work loses its gloss prematurely and, if examined under a magnifying glass, shows a slightly pitted or granular formation. This" deadening "-or "sinking-in," as it is usually called­results from varnishing over a graining colour which is underbound or too porous. Vandyke brown, in particular, if applied too heavily with in­sufficient vehicle, provides an absorbent surface which may contribute to this form of trouble.