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Paints for Exteriors

As outlined byBy Ray Lewhite. There's very much more to successful outdoor decorating than simply slapping on the paint. Make a first-class, long-lasting job of it this way....

Exterior painting presents quiet different problems from those associated with interior decorating -- different physical problems as well as different technical problems. If you decorate a room and don't like the result of your colour planning or some defect occurs because of insufficient preparation you can face the nuisance and do the job again at any time. If you paint the outside of your house and the paintwork breaks down, not only do you have to find out where you went wrong but it will be vital to put it right quickly and possibly at a time of the year when doing outside work will be very difficult because of bad weather and lack of daylight.

Protecting woodwork

Paints for exteriors
Even with the present and growing popularity of plastic/metal window frames there is still quiet a lot of woodwork on the outside of an average house and this needs protection from decay. The best treatment is still obtained by the proper application of paint. Taking as an example a semi-detached house of the three bedroom type we find that the doors may be made from hardwood but that the rest of the woodwork will be softwood. Of course the doors may also be of softwood and only the window sills made from hardwood and this is possibly oak. It is important to understand the different paint problems offered when dealing with hardwoods and softwoods and also to understand that quiet often the terms hardwood and softwood bear no relation to the physical hardness of the wood. It is a botanical distinction derived from cellular construction of the tree. This distinction is best illustrated by the fact that one of the most easily worked timbers, balsawood (easily cut with a knife) is in fact a hardwood.

A common difficulty

Paints for exteriors
This fact has considerable bearing on the adhesion of paint to wood as many will have discovered when dealing with windows which have frames made from softwoods but cills made from oak. Attempts to paint both frames and sills with the same paint system quiet frequently fail after a short time, a common problem among D.I.Y readers. In theory. and this can be borne out in practice too, a softwood construction can be protected against decay for quiet a long time by the following simple paint process; Well sand the wood (unpainted wood that is) with glasspaper and dust off. See that the wood is quiet dry and paint with a coat of oil-based wood primer rubbing this well into the wood rather than just laying it on. Next when the primer has had ample time to dry thoroughly, defects in the wood should be made good with a 2 pack wood filler, this will be ready to rub down in about 10 minutes. A coat of undercoat should then be applied and when this has dried (and has been allowed at least overnight drying) a coat of the gloss finish should be brushed on. This will then make a three coat system but an extra top coat will make for a better appearance and also add some weight and body to the protective coating.

Paints for exteriors
If I were painting a house from which all old paint had been removed I would consider that to be the minimum treatment for all woodwork calling for normal painting and I would be prepared to go further for better finishes to front doors and any parts known to get the full effects of bad weather. In fact I should be inclined to consider painting in terms of appearance of finish rather than in terms of number of coats applied, which may be variable in thickness anyway.
The foregoing assumes that the old paintwork has been removed and you are starting again from bare wood. It is not always necessary to burn off. To make a decision a careful assessment of the paint condition must be made. This is not easy for those who lack experience nor is it easy to describe in writing the various conditions of paintwork and at what point it can be re-coated and when it should be removed. There are obvious signs of course, areas of cracked and crazed paint, obvious flaking and peeling and large areas of old blisters. On the other hand it is possible for paint to look bad and yet be adhering well and have merely lost gloss and colour. In such cases careful preparation will give a good base from which to start building a new paint coating system. Before proceeding to methods of preparation let us go back for a moment to the reasons for painting and consider why it is necessary to use primer in some cases and not in others.

The important primer

Paints for exteriors
A primer is important in any painting of a multi-coat nature as it is the only coat of paint which actually touches the surface being protected and decorated. It has a double duty. It must adhere well to the base structure, either wood or metal. It must be capable of taking to its top surface the following coats of paint and allowing them to adhere also. In the case of bare wood treatment there are technical reasons why lead paints are considered good for priming purposes although there are other primers available nowadays which are considered of lead primers for this purpose. These will be mentioned later.
Where you get a painted surface, the paint film of which appears to be adhering well it may be assumed that the primer, even if type unknown, is doing its job well and subject to proper preparation, the new paint may be applied over the old coatings. It is also quiet in order to make good local bad places by preparing and re-priming these spots only and then building up with putty or wood filler to the original surface level.
I have said that one of the reasons for painting is to protect wood from decay, it being agreed that paint is cheaper and easier than replacement of timber parts which have decayed to the point of complete breakdown of structure, splitting and crumbling away and in general being very unsatisfactory. Another important reason for painting is to keep the moisture content of the timber at a fairly stable persentage in order to minmise the effects of movement as the wood alternately becomes wet and dries out. Even when painting is done it is sometimes difficult to prevent movement of timber due to moisture. We have all surely experienced sticking doors and windows through this trouble. Paint is not 100 per cent impermeable to moisture but for all practicle purposes and especially for exterior protection paint does a lot for wood, and the more paint the better the protection, providing always that adhesion is good. Quiet clearly the heavier the weight of coating the better it must stick.

Danger spots

Paints for exteriors
One of the weaker points in painting work is that ingress of moisture often occurs at places which are not seen and not accessible. These are such places where window frames, sills and door frames and lintels meet brickwork. It should be the responsibility of the builder to ensure that before fitting woodwork all end grain and parts which will be inaccessible afterwards are in fact coated with primer at least. Most builders do in fact see to this, but a lot of house joinery is factory machine made and arrives on the site in pink primer. The builder has no control or possibly even any knowledge of what paint has been put on the timber.
In point of fact machine-made joinery which arrives in a pink coating is almost sure to be painted with a lead-free paint as in production work the products will be sprayed for speed of production and as lead paints are never sprayed because of their toxicity one must assume that the paint is lead-free. The solving of paint failure problems is very difficult at any time and months afterwards it is practically impossible to be specific about the exact cause of a condition which will have been certainly the reason for the paint failure. In other words, although one can say dampness was the cause of paint flaking and peeling it will be very hard to judge where the damp entered. The excess moisture may have come from adjacent brickwork because of insufficient sealing or it may be that the undercoat or the finish coat was painted over a damp surface, primer or undercoat as the case may be. It would be very hard even for the expert to judge and certainly no one will suggest taking out door or window frames in order to check on the paints used at these points!
It does not follow that there will be paint failure every time woodwork is painted if the ends and edges of wood against bricks do not happen to be properly painted. Normally adequate ventilation and the ordinary damp course in a house will take care of excess moisture, the wet will evaporate in the normal manner. It is when poor position or design allows water to collect and seep in that trouble starts. I would, however, like to make it clear that where one has any control over woodwork being installed every effort should be made to see that the ends of verticle timbers especially should be painted. After all it is the very nature of a tree to take in moisture in a vertical direction and even when converted into man-made joinery the wood will be inclined to do just this to the detriment of itself eventually.


Paints for exteriors
Aluminium primers these primers made from aluminium powders in some form of oil/resin vehicle, have properties of sealing because of the leafing effect of the aluminium flakes and with the right kind of "stickiness" imparted by the liquid portion of such a primer will be very useful for woods which are close grained and into which it is impossible to force paint. Plywood is a good example of this and aluminium primers will be found better for these laminated boards than other types of primers.

Application of primers

Paints for exteriors
Not all paints are applied in the same manner oily white primers must have a different application technique from the aluminium primers. To be thoroughly effective oily white primers must not simply be laid on the surface of the wood but must be rubbed into the grain of the wood. This calls for a stiff brush to make the task easier, the primer need not provide a "solid" appearance although, after the initial rubbing-in part of the work, you can brush over lightly with the paint to give a coated look to the job. Drying time should be extended in this case as the heavier coating will obviously will require longer to harden off. In normal house painting overnight drying between coats is the rule and 24 hours will take care of most primers to the point where they may safely be coated over with a undercoat in cases where no stopping or puttying is necessary.
There is an exception to the rule regarding the application of oily white primers. Oak sills or any other oak woodwork which has to be painted with paint as distinct from varnish present a different problem. This arises from the nature of the wood. It is open grained and on the face of it one might imagine that it would be easier to rub in primer on this wood than on other, closer grained timbers. Unfortunately this is often far from the case and paint failures, on oak sills especially, are frequently encountered and might well be the source of some uncertainty as to what had been done wrong, particularly if one had taken great care with the priming work.
What happens is that air in the grain pores of the oak, small in amount though it may be, will effectively prevent the solid paint from getting right into the pores and the paint will in fact bridge these grain pores and there will be nothing to indicate that all is not well. The undercoat will go on quiet normally and the gloss coats will stand up well when applied. What may happen next is that the trapped air will expand in the heat of the sun and this expansion will probably be severe enough to blow out the paint film at these points to such an extent that the blisters burst and the paint film is ruptured. It is quiet easy to imagine what the effects of this will be, the weather will get in and in time a complete breakdown of the film occurs. There are several things you can do to counteract this state of affairs. First of all if such failure happens on reasonably new paintwork the damaged paint must be stripped off and a fresh start made. One priming method that can be tried is to use the primer quiet thin and use this thin paint as a first sealing coat rubbing well in and, when hard, next day or later of course, brush on another coat. Another method is to rub in varnish and see that it enters the grain pores and dries well, then follow this with your primer in the usual way, again rubbing in as effectively as possible. Aluminium primers depend for some of their properties on the leafing effect of the metal powders and thus a reasonably continuous coating is necessary. But this does not mean that the coating should be heavy. In fact no primer should be brushed on in a heavy coating.
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Special Paint Finishes.