Blistering is one of the commonest of painting troubles and one from which few decorators of any experience would be prepared to guarantee immunity, for it will sometimes take place on work which has been prepared and finished with the utmost care and on which only first-class materials have been used. In consequence of this, the reason of its appearance is not always easy to determine and may involve patient and careful investigation.
The trouble is liable to develop on both interior and exterior work, applied over wood, metal, or other materials. Heat encourages its formation, the direct rays of the sun being particularly damaging in this respect.
B1isterlng over Woodwork.Probably the two most fruitful causes of blistering are the presence of moisture or imprisoned air, either beneath the paint or between the various coats, and of resinous deposits; the use of undercoats which are too soft and oily increases the liability of blistering. Moisture. Under present-day conditions, a great deal of unseasoned timber is inevitably employed in building construction, often with disastrous effects on the paintwork If any attempt is made to paint woodwork which is obviously waterlogged, no proper adhesion of the paint to the surface will take place; the heat of the sun will draw the moisture to the surface and cause it to expand, pushing the paint film away from the wood in the form of a blister.
The woodwork, however, may appear to be dry on the surface, though it is actually damp beneath. In such a case, the moisture may penetrate from the back of the paint film and so weaken the priming coats that a faulty area is created at which, after a certain amount of air has penetrated, blistering will ultimately develop.
It is sometimes assumed that, when moisture is the cause of the trouble, the blisters, when opened, will invariably contain water and that, if they are only full of air, some other explanation must be sought. This is not the case, for although the presence of moisture may not have been responsible for the actual expansion of the paint film into blisters, it has brought about the conditions which have led to their formation.
A not uncommon manifestation of the defect may be observed on exterior woodwork, such as on oak gateposts, which have been allowed to stand for some time before being painted. Thus unprotected, the end grain of the wood readily develops cracks through which the moisture enters and becomes a potential source of trouble which needs special attention when painting is undertaken-a point which will be discussed later.
A likely cause of blistering is the imprisonment of air, by paint, in a porous spot, especially if the paint layers above it tend to be on the soft side. It should be realised that air expands with heat even more than does water, so that the least trace is sufficient to cause trouble if, when the temperature is raised, the paint becomes sufficiently soft to allow such expansion to take place.
Resinous Deposits.-Certain timber, as, for example, pitch pine, is"exceedingly resinous, and paintwork applied over it is the more liable to blister on this account. When the paint is subjected to heat, the resinous deposit is drawn to the surface and may even cause the paint covering it to " weep" or run right off. The smallest knots or pockets in woodwork, if not sealed, will have a weakening and softening effect on the paint, for as the resin melts under heat, it runs under the film, releasing air from within the pores of the wood, which becomes trapped behind the paint, thereby inducing the formation of blisters.
Blistering over Interior Woodwork.-In nearly every case this occurs on paintwork which is subjected to a high temperature, or where steam or condensed moisture is present. It is most commonly found on such positions as painted overmantels in living-rooms, or in kitchens and bathrooms.
Blistering over Metal Surfaces.-Iron or steelwork which is kept free from rust or corrosion should give little trouble when painted so far as blistering is concerned. Work which has been allowed to rust, however, is always subject to this defect, since the application of paint over loose scale or pits in the metal leads to traces of air being imprisoned. If painting has to be carried out during moist or rainy weather, the risk of blistering is increased.
Galvanised Ironwork.- The galvanising process leaves a coating in which there are microscopic fissures, some of which retain a certain amount of saline matter. The latter absorbs moisture from the air and is thus a potential cause of blistering unless suitable treatment, as described below, is given.
Investigation of Paint Blisters.-The first step in determining the cause of the formation of paint blisters on any job is to open up one or more of them and find out where the trouble has originated. In some cases it will be found that it goes right down to the bare surface, or in a repaint job where the old paint coating has not been removed, it extends to the latter, but occasionally it will be found between primer and undercoat or between undercoat and finishing coat. An investigation of this kind is thus useful, since it often limits the possible sources of trouble.
Its value is, however, qualified by two facts. The first is that blisters are not necessarily situated immediately over the faulty part, since the underlying gas and air may possibly creep to the nearest weak spot. To be on the safe side, therefore, not only the actual blister, but the area immediately surrounding should be laid bare if any making-good is contemplated. The second is that an old painted surface, which appears to be perfectly sound after having been rubbed down, may yet contain a number of minute blisters, hardly big enough. to deserve the term and almost invisible to the naked eye. The application of new paint may, however, soften these weak spots sufficiently to cause trouble.
Prevention of Blistering.- The conditions under which a good deal of painting has to be executed nowadays are such, unfortunately, as predispose towards blistering, and prejudice the chances of turning out good and durable work. The decorator has seldom any control over the surfaces which he is called upon to paint and much of the woodwork used in the construction of new houses is far from satisfactory from his point of view. Unseasoned timber is bound to shrink; joints gape, allowing water to enter more or less freely with disastrous effects.
When the decorator is in a position to do so, he should see that only sound and matured timber is employed, and that when new woodwork has to be painted, it should, so far as possible, be primed under cover. Mass produced door and window frames are usually primed before they leave the factory, but, in too many instances, the primer is little more than a thin wash of unsuitable composition, which offers but little protection and fails to fulfil the purpose for which it has been, presumably, intended. Matters are not improved by the practice which obtains on many housing estates of leaving stacks of such woodwork exposed to the sun and the rain for weeks at a time.
Window and door frames should, needless to say, be as carefully painted on the edges as on the faces before being fixed in position. In cheap work, this is sometimes omitted, in which event sooner or later blisters are bound to appear. Priming coats on new wood should be thin in consistency and as finely ground as possible, in order that not only the vehicle but a proportion of the pigment can find lodgment in the pores of the wood, and thus obtain good adhesion. It will be appreciated that if the pigment is too coarsely ground, only the liquid portion of the paint penetrates the cells of the wood, leaving an under-bound layer on the surface.
The use of successively more elastic coats, thoroughly well brushed out and allowed ample time to dry and harden right through, will also minimise the risk of blistering.
Knots and resinous deposits in woodwork will almost invariably cause trouble unless precautions are taken against them. Really bad knots should be cut out with a brace and bit, the resulting holes being carefully plugged with sound wood. Failing this, they should be burnt out with a blow lamp. The latter should be played on all sappy portions of the wood and any resinous exudation scraped off. Smaller knots should be treated with genuine shellac knotting. Any crevices which result from the use of the burning-off lamp should be filled with good-quality hard paste stopper.
Posts and unpainted woodwork in the grain of which surface cracks have developed need careful attention. If the fissures are really deep-as they sometimes are on oak gateposts-capping with sheet lead may be necessary. In ordinary cases the cracks may be stopped in the usual way, but to ensure adhesion of the stopper it is a wise precaution first to treat the surface of the wood with a mixture of equal parts of gold size and boiled oil, thinned with turpentine. When this is dry proceed with the stopper, pressing the latter in well with a trowel. Allow at least a day for this to become thoroughly hard and then apply primer and subsequent coats.
Use of Water Paint to Prevent Blistering.-A process sometimes recommended as a safeguard against blistering of paint on new woodwork, or wood which has been stripped of paint, is to use a good-quality water paint as a primer, instead of the more usual oil paint. Two coats of the water paint are usually advocated, followed by a sharp coat of paint and two coats of varnish.
The idea on which this system is based is that the water paint is less likely to be affected by the presence of moisture in the wood than is an oil paint and consequently will not tend to blister so readily. On woodwork which is only moderately damp, it may-and often does-fulfil its purpose, so far as the prevention of blistering is concerned.
But in most cases it will do so at the expense of the durability of the finish; a good primer for wood should have good adhesion, moisture resistance, and elasticity, and a water paint, of however high a grade it may be, possesses none of these essential properties.
Certainly it cannot compare with an oil primer in these respects, and since the life of a paint film depends in no small measure on the quality of its priming coat, it is unreasonable to expect a finish based on a water-paint foundation to withstand outside exposure for very long.
Where blistering is due, not to moisture in the surface, but to the presence of knots and resinous matter, the use of water paint as a primer IS likely to be even less effective; the coating it provides lacks the mechanical strength to hold back resinous exudations and is easily fractured by them.
Prevention of Blistering on Ironwork.- The observations made on the preliminary priming and protection of new woodwork apply with equal force to ironwork. The use of metal window frames and fixtures in presentday building construction has increased enormously of late, and although some manufacturers of these articles take care that their products are properly primed before they leave the works, others are very negligent in this respect, with result that rust soon makes its appearance.
Ironwork pitted with rust must be cleaned and all scaly matter removed. This will leave microscopic pits and pockets in the metal, and to enable paintwork to penetrate these and seal them against any future entrance of moisture, the priming coat should be fairly thin in consistency.
Remedy for Blistering.-In most cases, the only course to pursue when paintwork has blistered is to strip off the existing finish and repaint, taking the necessary safeguards, as outlined above, against recurrence. On occasion, when only one or two small blisters have appeared, it may be possible to cut these out and touch up, but such repairs are usually too obvious for most people's taste. If recourse is made to this practice, the parts exposed should be treated with pure shellac knotting before repainting.