How to Paint Greenhouses

There are few types of structure which depend more for their stability on paint than does a greenhouse, and few in which the atmospheric conditions are more severe; there are good reasons, therefore, why special care and attention should be devoted to their treatment.

The majority of greenhouses are constructed of wood, though in recent years an increasing use has been made of steel frames, while aluminium is also finding favour for the purpose. In most instances in wood-built greenhouses, the wood employed is deal, which is subject to fairly rapid decay if it is not properly protected by paint. Some use is made of Western Red Cedar, a wood which has great natural resistance to all forms of decay – so much so, that painting is considered by some to be superfluous. Some form of treatment of the rebates is desirable, however, since if they are left bare the wood is liable to absorb oil from the glazing putty which, in consequence, will tend to loose its adhesion. If the wood is left unpainted, moreover, it will be subject to a good deal more movement, according to the atmospheric conditions prevailing, and this again will impose a strain on the putty.

We will begin by considering the painting of a new deal greenhouse. A great number, especially of the smaller houses, are built in sections and delivered to the customer, either ‘ in the white ‘ or primed, to be erected on a suitable site. Whether one of this type is to be treated, or one is being built in the ordinary way, it is of the utmost importance that all sections should be carefully primed before assembly; if this is not done, parts of the work after erection will be inaccessible to the painter, and it is at these points that moisture will subsequently find an entry. A first-class wood primer, such as a genuine white lead with a little red-lead paint, should be employed, special attention being paid to end-timbers which should be double-primed.

Glazing

It is often the custom to carry out the glazing when the structure has been primed and erected, but better results are obtainable if the glazing is deferred until another coat of paint has been given. It should be borne in mind that once the glazing is completed, it will be difficult for the painter to reach some parts of the work, so that it is desirable that the priming should be reinforced by a further coat of paint as a barrier against moisture which may later find its way in.

Only genuine linseed-oil putty should be used for the glazing; the practice of top-puttying – that is, of running a bevelled layer of putty over the edges of the glass and the glazing bars, as in domestic work – is not to be recommended; the objection to this system is that exposure to the rays of the sun will, in time, cause the putty to shrink and lose close contact with the glass, thus allowing an entrance for water. A better method is to place the glass on a good solid bedding of putty, press well home, and then cut on all superfluous putty level with the glass.

After glazing, give at least two more coats to the outside woodwork and one to the inside. The paint should extend on to the glass by not more than about one-third of an inch; it must overlap to about this extent, in order to provide an effective seal, but it should not be cut in wider, for it must interfere as little as possible with the passage of light and sun to the plants inside.

The choice of paint for the final coat is important, bearing in mind the conditions of warmth and humidity inside the house. A paint which will retain its elasticity, will dry with a hard, smooth film, and will not tend to chalk excessively is wanted. A type of paint frequently employed for the purpose, and one which gives good service, is a composite mixture made from approximately 75 per cent, white lead to 25 per cent, zinc oxide, so far as the pigment portion is concerned. Many manufacturers supply a proprietary product of this nature, but the decorator who prefers to make up his own can conveniently do so by adding a small proportion of a good long-oil enamel to a genuine white-lead paint.

Repainting

In renewing the paintwork of a greenhouse, special consideration must be paid to the condition of the coating, both inside and out. It is useless repainting the exterior surfaces if the interior paint has perished, for in this case atmospheric moisture within the structure may push the outside paint off from beneath.

If the adhesion of the existing paint film is at all doubtful, it is safest to strip down to the bare wood. If repainting has not been carried out for some time, and the coating has perished to any extent, it is probable that a certain amount of moisture has entered the wood and the first objective is to eliminate it. The best way of doing this is to remove all paint from both the inside and the outside and leave the greenhouse open to the sun and the wind for some days in fine weather.

All loose putty should be hacked out and the rebates repainted and re-puttied. It is a waste of time and material to paint on top of putty which has lost its grip, for the heat of the sun will cause it to curl away still farther from the glass, splitting the new paint film in the process. Any paint which abuts on to the glass should be carefully inspected. In places, it may be found that though it appears to be intact, it has actually lost contact with the glass, and the slight contraction brought about by the drying of new paint on top of it may cause it to draw away still farther.

If the owner has not kept the glass free from dirt, the opportunity of cleaning it should be taken. In most cases, a mild solution of sugar soap will prove effective or a solution of American potash (about 1 lb. to 5 pints of water); thorough rinsing in clean water should follow the use of either of these media. In bad instances, neither of them may prove sufficiently strong to remove the accumulations of years and, as a last resort, a weak solution of hydrofluoric acid may be necessary. This must be used with great care owing to its corrosive nature and rubber gloves should be worn; if applied too strong or left on the surface for too long, it will etch the glass. Only a small area should be treated at a time, by means of a mop, and washed off quickly with clean water. The acid must not be allowed to come into contact with any paintwork or metal, which will be badly affected.

Atmospheric conditions in a greenhouse encourage the development of mould or fungoid growths; these may be of the ordinary black variety or, occasionally, of a pinkish-purple colour. The latter is not always easy to detect and may, superficially, look like a discoloration of the paintwork, though, under a powerful magnifying glass, the characteristic bunches of mycelium can be clearly detected.

Many gardeners are in the habit of using chemical washes for their plants and some of these preparations may contain sulphur. If lead paints have been used, this may have caused the surface to blacken in parts, though the protective properties of the paint are not necessarily affected thereby. A solution of peroxide of hydrogen will usually bleach out this kind of discoloration.

In heated greenhouses, the pipes are often left unpainted on the ground that a coat of paint interferes with efficient heat radiation. Provided that a dark-coloured paint is used, there will be very little loss of heat, and some form of coating is desirable as a protection against corrosion. Tar or bituminous products are often employed for the purpose, but tend to craze: their use is also open to the objection that a few delicate plants – e.g. some species of ferns – may possibly be affected. A good-quality heat-resisting black is probably the best material to use.

Steel-frame Greenhouses

The painting of steel-frame greenhouses differs little in principle from that of other iron or steel structures. The chief consideration is to protect the metal from corrosion; this involves careful cleaning and preparation of the surface and the application of a good rust-inhibitive primer.

Ordinary linseed-oil putty is unsuitable for use on steel-frame greenhouses as the metal is non-absorptive and the putty in consequence has poor adhesion and takes too long to dry. Many firms supply putty suitable for this class of work and this gives the best results. Some painters add a little gold size to linseed-oil putty, to speed up the drying or, alternatively, add about 1 oz. of red lead to 1 lb. of putty, to serve a similar purpose. These expedients bring about some improvements, but neither putty is as satisfactory as one specially formulated for this class of work.

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