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Some Facts about Undercoats
By Ray Lewhite.

In a earlier article I discussed the essentials of priming woodwork and the types of primer in use and emphasized the necessity of rubbing in primers rather than merely laying-on the paint.
The method of preparing woodwork to take primer may be either :
(1) glass-papering after burning off or with new woodwork.
(2)glass-papering following removal of paint by means of a paint remover or a spiritous or paste nature and in the case of sanding should be preceded by a washing with a solvent such as white spirit.
(3) Preparing damaged patches on paintwork which is otherwise sound and this preparation will normally be scraping away the loose paint flakes and rubbing with glass paper. The rest of the paint should be well rubbed down.

Some facts about undercoats
When all paintwork has been prepared and primed, or patch primed, as the occasion demands, attention should be turned to the next stages of the paint preparatory work, which is the making good of surface defects, the applying of the undercoat and the finishing coat or coats.
There are two sound reasons for making good surface defects. The first is for the sake of appearance whilst the other is in the interests of the protective value of the painting work. Nail and screw holes, splits, cracks and crevices in wood fabrications all detract from the appearance of the paint job as does rough surfaced timber which has not been planed properly. Grain markings of a pronounced kind will show through coats of paint up to three or four in number unless something is done at the preparatory stages of the painting. This may not be very important on upper storey woodwork but should really have some attention on ground level joinery such as front doors.
As the purpose of painting exterior woodwork at all is to preserve it from the ravages of the climate, it is reasonable to assume that the smoother the surface of the paint the less chance there is of water lodging or of dirt becoming embedded at rough places. Smooth surfaces of pleasing appearance offer encouragement to those willing to carry out occasional maintenance in the way of washing down and polishing with wax polishes the more accessible parts.


Some facts about undercoats
It should be a basic rule of painting that apart from actual woodworking repairs all making good of paint surfaces must be done after the primer. Again there are two sound reasons for this ; the first is that all stoppers and putties have their efficiency conditional upon being applied over a coat of paint and the second is that after a coat of primer has been applied the defects are easier to spot.
All stoppers and putty of the conventional type and excluding those of a plaster and water mixable kind are based on some form of powder bound with a varnish type of material or an oil. Linseed oil putty which is used for glazing as well as for making good defects in wood work consists of finely sifted whiting bound with raw linseed oil.
In this form the putty will dry slowly if applied over a previously painted surface, but if forced into a crack in woodwork or used for fixing glass where there is no paint on the wood bars there is every risk that the bare wood will drink up the oil leaving in time, the powdered whiting very much as it was before mixing into putty form.
Linseed oil putty has certain disadvantages when used in straight painting work. It is capable of being applied and levelled off with the knife quiet easily but paint applied over it will still be soft for quiet a long time after application and any attempts at rubbing down will almost certainly disturb the filled surfaces.
Its use in exterior painting should be confined to places unlikely to be touched after painting and all upper storey painting can with safety have linseed oil putty used for making good defects. The levelling of rough grain and other shallow defects and the filling up of deep defects likely to be worked ove3r during painting should be done with something rather more tractable and of a stickier nature as well as quicker drying.


Some facts about undercoats
There are several proprietary fillers now available which are easy to use and can be worked on to surfaces with the object of local filling and making good.
It is sometimes better to apply stoppers and fillers and leave them "proud" and rub down afterwards rather than attempt a smooth level finish the first time over with the knife. Also you may find that two shallow applications of filler are better and will dry much quicker than one deep layer..
As an alternative to the fillers made up on the job with water you might find it preferable to make a stiff stopper from the undercoat paint and filler in the same manner in which linseed oil putty is made.
I have found that there is an advantage here especially when some small defects have been found in the later stages of the painting work and one is unwilling to have to use a stopper of a different colour and then patch it up with undercoat to get an opaque colour effect.
For stopper and putty application a good putty knife and a good broad filling knife are worthwhile investments especially if neither is at anytime used for scrapping work. You may find that some wood surfaces look rough and feel rough to the touch without the roughness being deep enough to take any kind of filler or putty. An improved surface for the final coat or coats of paint may be achieved in such instances by brushing on a rather heavy coat of undercoat in addition to the one which will be given to the entire working area but some rubbing down may be needed especially on important parts of the work.


Some facts about undercoats
Normally one will purchase the undercoat and the gloss paint from the same maker. The two paints will be complimentary, the undercoat serving to promote build and thickness to the finished paint film, help with obliteration and drying and generally be part of a successful paint system. The gloss paint giving the top protection against weather, glossy appearance but not much in the way of obliteration.
I emphasized that primers should be well rubbed in rather than laid on. With undercoats its a different matter, a reasonable full coat is desirable. It is difficult to describe what a reasonable full coat of paint is, but the aim should be obliteration of the paint or primer underneath, a surface reasonably free from brush marks caused by a dragging brush and insufficient paint, whilst a too liberal application of the undercoat will show itself in subsequent runs or sags. Although i have said that in undercoating one should try for complete obliteration, this is not to be done at the expence of an even coating of paint as too heavy a coat of undercoat, even if no runs occur, will stay soft too long and may hold up the progress of the work later, especially if some rubbing down is desired to get a better finish.
Runs and sags are equally detrimental to the life of a paint film not simply because of the poor appearance but mainly because of the uneveness of the paint film. The thickness of a paint coating at the point immediately above a gathered run will be very much less than at any other place and the risk of breakdown will be greater at such places than where the film is even in thickness. It is true that in a multi-coat paint system on wood the risk will not be as great as would be the case with a one or two coat paint film on metal prone to rust.
So we are apt to deplore run and sags in paint work on the grounds of appearance only and on those grounds we must consider how to avoid them in applying paint.
The first thing to do is get your paint at the right viscosity or consistency. Many paints are described as being ready for use but this readyness will not always be apparent, especially when the temperature is low, the paint will then be too thick for easy brushing and some thinning will seem desirable. Only a small amount of white spirit will be required to adjust the paint to a brushable consistency.
Over-thinning can lead to uneven coating and the forming of runs just as easily as having the paint too thick. In the first instance covering power will be poor and one applies more paint to get obliteration and in the second instance, the paint being heavy cannot easily be brushed out to an even film.

The choice of the right brush for the particular surface area being painted is the next step towards good even coating.
Two brushes are normally in action at one time, one a 1 inch used for glazing bars and narrow sections of wood, the other could be a 3 inch or smaller for all wider areas and panel surfaces. The general rule is to always use the large brush whenever possible reserving the small one for places obviously impracticable for the large brush. I have seen beginners whose work was obviously doomed to failure at the outset, irrespective of whether or not the paint was correctly mixed, because they were attempting to paint large areas with small brushes.
The actual technique of brushing on paint is not hard to learn but does call for practice like all other aspects of craftwork. If we take as an example an area such as a door panel the correct manner in which this particular panel should be painted will be as follows : With the inch brush loaded with paint the outer edges of the panel are painted in, this will leave a band of paint about 1in. to 2in. wide all round the edge of the panel. Next charging the 3in. brush with paint we paint in verticle bands of colour at alternate brush widths across the panel. Thus if the brush gives us a band of paint 3in. wide we start at 3in. from the edge of the paint laid on with the small brush and so on. Our sketch makes this clear.
The verticle bands of paint are then joined up without applying any further paint to the brush, and in fact the brush must be scraped against the edge of the paint can to free it from excess paint before proceeding to spread out the paint applied to your panel area. This joining up is followed by brushing in horizontal fashion across the panel to spread out the paint as evenly as possible and the final strokes are again verticle. This is called laying-off.
It may be imagined that the painting procedure as described is lengthy and may lead to the paint setting up before one has finished brushing. This will certainly be the case if the brushwork is done very slowly but in practice when the art has been mastered it is quiet as quick to work in the methodical manner described as in any other and runs and sags will be virtually eliminated.
The method i have suggested for dealing with large areas can be practiced before attempting anything important and of course this only deals with simple panel areas. In practice there are snags which the inexperienced will have to watch out for, such as how to deal with running paint from the top edges of doors, whether to paint quirks and moldings first or to paint in panels and then deal with moldings.
I cannot deal with all these points in one article but i propose to say more about them in an article dealing with the getting of good finishes. Apart from special places requiring two coats of undercoat, or where one coat will simply not give the opacity required, one good coat of the undercoat paint should bring your exterior painting work to a state of readiness for the gloss enamel paints. I will deal with the special problem off getting up rough places to a reasonable finish later and will keep to the step by step coating system now.


Some facts about undercoats
Where the exterior paint scheme consists one colour only the work from this stage calls for two coats of gloss paint. One coat is enough to give reasonable protection but in my opinion two coats of gloss will be better both in appearance and durability. For a two colour scheme you will wish to apply one coat to the chosen area and then re-coat when dry before proceeding to the second coat.
If the scheme calls for cream to window glazing bars and the remainder of the woodwork in some darker colour then naturally you will do the cream parts first. If the area to be cream is quiet small a single coat, providing it is "solid" can be reckoned to be sufficient and you can confine the two coat work to the greater painting areas.
With two coat work the first coat can be applied more sparingly and can even be thinned a little to make for easier working under the brush. The final coat however, should be as full as you can make it, bearing in mind the remarks about runs and sags.


Some facts about undercoats
Most painting work done by houseowners must of necessity be done at varied intervals of time. This is an advantage sometimes as it means that paint which should be hard before the application of another coat can be allowed the proper time, but i must emphasize the risks attendant upon leaving undercoated work too long before applying gloss paint.
The undercoat paints are formulated so that they have a beneficial effect on the gloss paint which should follow fairly soon after the undercoat has dried. If the undercoat is left to long before applying the gloss coat or coats it may be so hard as to be glass-like and offer poor adhesion to the gloss enamel paint. Poor adhesion leads to early breakdown in the form of flaking.
I would say that a fortnight would be the maximum period i would care to leave undercoats before folowing on with the gloss paint. This means that one should asses the probable weather possibilities along with the time available and only paint with undercoat those areas you can be sure of completing within a fortnight or so. Where this is impossible to ensure then it will be necessary to roughen the surface of the undercoat slightly with glasspaper and for preference apply, another thinner coat before the gloss paint.
In fact this kind of planning is necessary with all aspects of exterior painting which must be done on a piece by piece basis. For example : when burning off and priming you should burn off, scrape clean and glasspaper and apply primer to just as much work as can be managed that day or working period. Leaving burnt off and unprimed wood to the mercy of the weather for a week or so is not very wise. Similarly with painting. One must see that wet edges are caught up and complete areas painted even before breaking off for a meal.
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Special Paint Finishes.