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Decorating Defects - their cause and cure
Cracking of Paintwork
Cracking is one of the most common forms of failure and one of the
most serious, for its occurrence frequently foreshadows the breaking up of the paint film and implies that the latter is not fulfilling a primary function -the protection of the surface to which it has been applied.
The cracking of paintwork which has weathered for a reasonably long period is due to the film having lost its elasticity and being, in consequence, unable to accommodate itself to the movements of the surface, following atmospheric changes. Paints which dry to a hard and brittle coating are more likely to exhibit this tendency after a comparatively short life and it should be obvious, therefore, that flexibility is an essential property of any good exterior paint.
Premature cracking can usually be traced either to the use of unsuitable materials, to incorrect coat sequence, or to failure to allow adequate drying time between coats. The application of short, hard-drying paint or varnish over an oily coating will induce cracking because the short coat lacks the elasticity of the latter which will contract and expand to a greater extent than that of which the top coat is capable. A good example of this
may be seen when bleeding has occurred in a white or pale long-oil enamel finish and an attempt is made to seal the discoloration by the use of a coat of shellac knotting; the latter, being far more brittle than the enamel, will
induce cracking.
It may be objected that an undercoat containing very little oil is
almost invariably applied over a far oilier primer, without ill effects. This
is true, but allowances must be made for the fact that a fairly high propor
tion of the oil in the primer is absorbed by the surface underneath.
In hardening, films of paint or enamel contract in a certain degree; it
is therefore important that each successive coat should be progressively more
elastic; if this precaution is not observed, there is a strong risk that a coat
will shrink to a greater extent than the one over which it has been applied
and to which it adheres; this will set up surface tension which, if too great,
will result in cracking.
For a similar reason, ample time must be allowed for each coat to dry
before another is applied on top of it. Ira film of paint or varnish is covered
up in this way before it has had long enough to harden all through, its supply of oxygen is cut off and the drying process is checked, leaving the
lower portion of the film still relatively soft and consequently more elastic
and liable to movement than it should be. At times, such movement may
do no more than cause slight wrinkling of the finish, but there can be little doubt that on many occasions it will bring about actual fracture.
In bad cases, when the fissures extend right through the paint system,
the only treatment is to strip the coating with paint remover or solvent, or
by burning off, and refinish from the beginning again, and this method is probably the cheapest and certainly the safest even if the trouble is con
fined to only one or two coats. It is sometimes possible, by the use of a
solvent-type paint remover, to strip only the coats in which cracking has
taken place, but the operation needs care and considerable experience and
may take far longer, on a surface of any size, than the complete removal of
the coating.
Alternatively, if only the upper coats are cracked, it may be practicable
to cut down the surface by rubbing with an abrasive, fill the cracks with
hard stopper, pressing well down with the knife, and removing any excess
material, and to refinish in the ordinary way. Care should be taken before
applying the paint to rub down the stopped areas so that they are in the
same plane as the rest of the surface.