Preparing Surfaces for Painting: Stopping and Filling

Stopping and filling can conveniently be considered together, since basically they fulfil similar functions – the levelling up of inequalities in a surface before the application of the finish. Stopping is concerned with cracks, crevices, bad scratches, nailholes, and the like, whereas filling is carried out to build up a level surface by forcing suitable materials into indentations, however small these may be. Stopping is essential where the defects mentioned exist in the surface, whereas filling is frequently omitted in all but good-grade work.


Various mixtures are used for stopping, but if they are to be satisfactory they should possess certain properties as, for example, spreading or knifing qualities, reasonably rapid hardening without shrinkage, and a porosity approximately equal to that of the surrounding surface.

The spreading properties of a stopper depend largely on its consistency and adhesive qualities. It should be stiff enough to resist pressure, a great deal of which will be necessary at times in forcing stopper into deep holes or cracks. At the same time, it should be capable of being worked under the knife when shallow depressions have to be levelled up. The hardening of a stopper is largely dependent on the thickness in which the latter is used. By employing a minimum of binder, a reasonably quick-hardening product may be obtained, but it is inclined to be somewhat porous and should not be used for large or deep holes or crevices; it is better to use a better-bound mixture, applying it in two, three, or more instalments, according to the depth or extent of the cavity.

Stopper, when set, should be neither harder nor softer than the fillers, undercoats, and other materials applied over it which need to be sandpapered or flatted down; if it is harder, there is a tendency, when rubbing down takes place, for the softer material immediately over or around the stopped area to ‘ fret’ away too readily, leaving a ridge around the edge of the hardened stopper; conversely, if it is softer, the stopper is inclined to rub away, leaving a depression.

For obvious reasons, the absorption of the stopper should be as nearly equal as possible to that of the surface on which it is used. A precaution which might well be more widely observed than it actually is, is to match up the tint of the stopper to that of the remainder of the surface; in the days of multicoat finishes, this was comparatively unimportant but nowadays, when the minimum number of coats is applied, failure to take this simple precaution may often result in the stopper ‘ grinning through,’ involving the use of an extra coat.

Stopping should always be carried out on woodwork after the latter has been primed; if it is executed on the bare wood, there is a risk that the oil or binding material in the stopper will be absorbed by the woodwork, leaving the stopper without sufficient to ensure adhesion and more liable, also, to shrink after application.

For nail-holes and similar defects in everyday work, the form of stopper most popular among painters is ordinary linseed-oil putty. It is cheap, easy to use, and sufficiently plastic to accommodate itself to subsequent movements of the wood, but it cannot compare with hard stopper, made from paste white lead, whiting japan, gold size, and turpentine or white spirit. A typical recipe. If desired, a little red lead can be added to the mixture to harden it and speed up the drying.

A considerable amount of force must be exerted to drive the stopper home in deep holes and depressions; this should be done in stages, without using too much material at a time, or air may be imprisoned with the stopper, with the result that later, a certain amount of sinking will take place. Any surplus should be levelled off with the knife, though some men prefer to leave it projecting very slightly beyond the plane of the surface, so that, when it has hardened, slight sanding will take it off flush.

When the woodwork has to be finished in stain and varnish, instead of paint, even more care is needed in stopping, owing to the transparent nature of these materials. It is important that the suction of the stopper should be much the same as that of the woodwork, because if it is different, it will take the stain differently and this variation in appearance will tend to be emphasised rather than concealed by the varnish. Stoppers for this type of job vary greatly according to the quality of the finish. For cheap jobs, it is fairly common to touch in the hole or crack with size and fill with plaster of Paris, bound with a little glue size and roughly tinted to match the wood. This process can be recommended only on the grounds of cheapness. Pea flour, or a mixture of plaster of Paris and pea flour, bound with glue size, are other stoppers favoured for inexpensive work; within limits, they take the stain and yield to sanding quite well. For better-class work, a white-lead gold-size stopper is to be preferred or, alternatively, a good plastic wood; the latter is available with a cellulose medium which has the advantage of drying hard, ready for sanding, in a comparatively short time.

Making-good on Plaster

Much of the stopping carried out by the painter will consist of making-good cracks in plaster – a job which, on the whole, is not done with much care or skill. Very often, Keene’s or some similar plaster is advocated for the purpose, and while it is possible to get quite good results with them, better ones are obtainable with one of the plasters specially formulated for patching; generally speaking, these are of finer texture and give a denser mixture than ordinary wall plasters.

Poor workmanship is often the cause of inferior stopping. A common mistake is to make up too great a quantity of the patching plaster beforehand, with the result that by the time it comes to be used, it has partly set before it is placed in the fissure and consequently forms a tight stopping. Cracks should be well raked out at an angle, so that a V-shaped section is left, and the work well wetted. Insufficient damping of the surrounding plaster and an attempt to fill too deep a cavity in one operation are frequent faults. The application of paint or any other impervious coating should not be carried out until areas which have been stopped are dry and hard or they will swell and be very apparent. Another point to be watched is to equalise the suction of the making-good with that of the rest of the plaster: as a rule, patches tend to be rather more porous than the existing surface, and because of this there is something to be said for the pencilling in of the affected areas with a thin coat of knotting; however, the absorption of plasterwork varies to such an extent that no hard-and-fast rule can be laid down.


It is to be regretted that the filling operation is nowadays usually omitted from all but good-class work, for, although it adds substantially to the cost, it makes a great deal of difference to the quality of the finish.

Fillers may be either of the stiffened distemper, or the oil type. For the former, various mixtures are used. Many of the proprietary brands of washable water paint, sold in paste form, are excellent for the purpose, the paste being slightly thinned with water. Ordinary size distemper, with the addition of a little fine plaster, is preferred by some; this must be well-bound without being over-bound, or there may be some risk of cracking. There are also numerous brands of factory-prepared fillers which, if made by a reliable firm, can be recommended.

There are two methods of applying distemper fillers. The first is to put on the material by brush and knife, over the whole of the surface, and then rub down until a level ground is obtained, and the second, to apply paste filler with the knife, leaving the work as smooth and clean as possible, thus reducing the rubbing-down to a minimum. For mouldings, it is generally necessary to use a brush, care being taken that grooves and angles are not choked up.

After application, the distemper filler coat is cut down by glasspaper, dusted off, and then oiled in, using raw linseed oil with a little drier, equal parts of varnish and turpentine, with or without a small proportion of linseed oil, or a thin coat of oil paint. The distemper coating is thus converted into a kind of oil coat, but care should be taken that the filler coat, while not too porous, is sufficiently so to absorb the oil or varnish coating.

Provided that distemper filler is made and used by a man who knows his job, it is durable and satisfactory in use and is relatively cheaper than hard filler, since a good deal of time and labour can be saved by its employment.

Hard Fillers

The old slate-powder type of hard filler, as employed by coach-painters in the past, is seldom used nowadays since it is too costly a process for most kinds of work. It consists of finely ground slate powder, bound with gold size and thinned with turpentine to paint consistency, and usually applied by brush. Two, three, or more coats may be required to produce a level surface; to ensure this, it was formerly the practice to apply a sharp coat of colour after the required number of coats had been put on, as a guide to rubbing down. If this colour-coat was removed all over the surface it showed that the latter was level; if any indentations had been left, the pumice stone would ride over the colour, leaving it intact and revealing that, in these areas, still more filling was needed. Such methods, followed by craftsmen, produced superlative results in the old days, but few jobs to-day would permit them.

When hard filling is employed it consists more often of a composition based on white lead – in many instances, a mixture of paste white lead, whiting, and gold size. White lead takes some time to harden but it is very durable and yields an elastic coating, provided the gold-size content is kept within reasonable bounds. A good mixing varnish may be used in place of the gold size, though the mixture will then take rather longer to dry.

Hard filling is best applied in stiff paste form by means of a knife, though it can be thinned down and brushed on. It must be given plenty of time to dry and harden before rubbing down; there should be no excess of binding material, or the filler may tend to rub up under the abrasive; however, if deficient in binder it will disintegrate and rub away too easily to serve its purpose.

Fillers for Metal

In most kinds of metal work there are surface imperfections which may not be very noticeable on the bare metal but show up badly when a full-gloss finish is applied. Castings are usually filled after priming in the usual way although, in some instances, blemishes are made good on the bare metal in the foundry. A mixture of iron filings, plaster of Paris, whiting, and Portland cement is often used for filling such surfaces, but this is outside the scope of the decorator’s work.

While most of the iron or steelwork which he is called upon to paint does not require a very high standard of finish, so far as appearances are concerned, and consequently does not need filling, he may sometimes be asked to refinish radiators or exposed pipes with a very rough surface on which filling is practically essential. Ordinary paste filler would not be suitable for this purpose, since the oil content would be affected by the heat, but most of the leading manufacturers make a range of primers, undercoats, fillers, and finishes for this type of work and these should always be used for this type of job.

Sealing Surfaces Liable to Bleed

Certain pigments – notably a number of red lake colours, stains of the creosote class, and bituminous products – become soluble when painted over and are liable to ‘ bleed ‘ or discolour white or pale coatings applied directly on top of them. In such cases it is quite useless to attempt to obscure the discoloration with a further coat of paint, for it will persist in showing through half a dozen or even more. The accepted remedy is to cover a ground of this kind with a spirit varnish, shellac being generally used for this purpose, since it is insoluble both in linseed oil and turpentine and thus effectively prevents contact between the pigment or stain liable to bleed and the new paint.

Grounds from which it is suspected that there is a risk of bleeding should therefore be tested, before any paint is applied, by brushing a little of the latter to an unimportant part of the surface and seeing if any change of colour occurs during the drying process. Such a change may take place a week or even more afterwards, the time which elapses before any sign of bleeding makes its appearance varying according to the hardness and condition of the groundwork, as well as to the nature of the new paint. Old, hard grounds are not so readily softened by the new coating, and thus it will take longer for the discolorations to develop. Again, the pigment or dye in the groundwork may be soluble in oil or turpentine, or in both, and may be more soluble in one than in the other, so that the ingredients of the new paint must be taken into consideration. If the latter happens to be of a quick-drying type, there is less danger of ‘ bleeding ‘ taking place.

It is seldom convenient for the painter to wait as long as a week to make the test, and a fairly reliable – though by no means infallible – method of speeding it up is to make up a white paint to normal brushing consistency and thin it with equal parts of oil and turpentine. Paint a little of this over the suspected ground and at the same time apply some of it over another ground, of a similar colour to the first, but which is known to be non-bleeding. Compare the two samples after about twelve hours, and if bleeding is to occur, some signs of it should be evident.

Bleeding may occur when painting over an old distempered ground if the pigment in the distemper happens to be soluble in either oil or turpentine. It is prudent to regard all red grounds with suspicion and to bear in mind that such shades as peach or apricot may cause trouble if a bleeding type of red stainer has been employed in mixing them.

It sometimes happens that signs of bleeding make their appearance only after one or more coats of white or pale-coloured paint have been applied over the affected ground. To try to remedy the condition by the application of a coat of spirit varnish on top of the new paint is to run the risk of trouble of another kind, for to superimpose a brittle and inelastic film of spirit varnish on a relatively soft and flexible coat of oil paint is to violate the rules of coat sequence and incur the liability of cracking. There is less chance of this taking place if only an undercoat has been put on, since this will probably contain only a small proportion of oil, but the danger still exists.

Applying The Undercoat

When the work has been primed and any necessary stopping, filling, or sealing properly carried out, one or more undercoats will be applied.

Recipes for undercoats and their composition will vary according to the nature of the finish or, in the case of old work, to that of the ground. Generally speaking, for a glossy finish, they will contain less oil than the primer and more pigment, so as to dry out flat or nearly so, since their main function is to obscure. They must, however, be sufficiently well bound not to rob the final coat of its oil, or its gloss and durability will be adversely affected.

In applying an undercoat over existing paintwork, due regard must be given to the condition of the latter, for though it may be adhering well, it may have chalked to such an extent that it has become unduly porous. In such a case, a rather greater proportion of oil than usual should be added to the undercoat, in order that the suction of the surface may be adequately satisfied.

Undercoats should be brushed out well to a uniform coating and should not be applied in too thick a coat. There is often a tendency to do this when, on old work, a dark-coloured ground is to be refinished in white or a light colour, or vice versa, but this should be resisted. If the undercoat does not obscure properly in one coat of ordinary thickness, it is far safer to apply another, instead of attempting to obliterate the underlying colour in one application. The finishing coat will, of course, assist in obscuring, but this is not its primary purpose and too much should not be expected of it in this respect.

While it is an advantage for undercoats to be of a somewhat similar colour to the finishing coat, they should not match the latter too closely or there will be some risk that, in applying the finish, ‘ misses ‘ may occur. For black, green, or blue finishes, a grey undercoat is generally employed; for other colours, it may conveniently be tinted a paler shade of the finish.

For most purposes, a white-lead undercoat serves very well, though it should not be used directly under a coat of white enamel, since it increases the tendency of the enamel to yellow on exposure. When a proprietary brand of finish is being employed, it is better to use the undercoat recommended by the manufacturer, if only because, in the event of premature failure of the finish, the makers of the latter may conceivably disclaim responsibility if any undercoat other than that advocated by them has been used.

Finishing Coats

The function of the finishing coat is to provide the necessary colour, the required degree of lustre, and a film capable of a reasonable amount of resistance to normal wear and tear. On outside work, where it will have to withstand the weather, it will be of an elastic nature and contain a high proportion of oil and /or resin to pigment, as a result of which it will possess a certain amount of gloss; on interior surfaces, it may range from a highly lustrous coating to one which is flat or nearly so.

Whether or not the final coat fulfils these requirements successfully will depend, to a very great extent, on the preparation of the ground, the nature of the undercoats, and the skill with which they have been applied. They must provide a good and smooth foundation to which the finishing coat will adhere properly, and which will not absorb so much of the oil or vehicle of the latter as to leave it insufficiently bound or, in the case of a gloss finish, to detract from its lustre.

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