Finishing of Plasterboard
Finishing of Plasterboard: A great article from down under
A plasterboard wall or ceiling presents to the painter with a surface comprising two separate materials of differing texture and porosity. For a satisfactory finish (one in which these factors are not telegraphed through the paint film) the differences in surface characteristics must be addressed.
Plasterboard manufacturers recommend finishing plasterboard by roller application of a proprietary plasterboard sealer over the entire surface in order to minimise texture & porosity differences and also migration of chemicals to the paint surface, thus avoiding sheen staining, and providing better concealment of joints, before applying topcoats. Choice of colours is important, as darker topcoats tend to highlight variations. White or light colours give greater total reflection and are particularly recommended for ceilings.
PAINTING OF PLASTERBOARD
If glossy paints are to be used in rooms of a large open plan or with uncurtained windows, then the highest level of finish is essential. (Even then it may not be possible to completely disguise surface variations). However, if cost factors preclude this investment, then very careful consideration must be given to:
The placement of windows and curtains
The type and position of lighting
The standard of paint finish
The positioning of both butt and recess joints.
Please note: Horizontal joints are recommended for walls with areas of severe glancing light.
PAINT SYSTEMS
Paint systems are available in varying qualities. The effect of using low quality paint, particularly for the sealing coat, may not be fully appreciated until after the system is repainted and cracking & peeling occur.
A three coat paint system,- typically one coat of sealer followed by two top coats, is the minimum specification recommended by plasterboard manufacturers.
Two coat systems are not recommended, however common they may have been in the industry, as they will almost invariably fail to disguise the joints and variations in porosity and texture, as well as having limited performance and durability.
Two coat paint systems comprise either one coat of sealer and one top coat or, worse, two coats of finishing paint alone.
PREPARATION & APPLICATION
The following comments should be taken as a guide for the preparation and paintings of plasterboard.
All surfaces should be clean and dusted down prior to painting,
First coat should be a roller applied proprietary plasterboard sealer - lightly sand when dry. A solvent - borne sealer should only be used if the paperfacing has yellowed after extended exposure to light, or where wallpaper is to be applied.
Apply two top coats (undiluted) - paint with roller, or brush for detailing.
It has been a long-standing recommendation of plasterboard manufacturers, that roller application of paints is preferred, as the light texture imparted by the roller improves the parity of texture over the entire surface.
GLOSS PAINTS
Both gloss and semi-gloss paints on flat surfaces reflect light in a pattern distorted by minor variations of the surface. This effect is enhanced as the observer moves around the room. Flat paints, on the other hand, tend to reflect light back in a diffused form, which does not draw the attention of the observer to minor surface variations.
It should be noted that plasterboard is manufactured on a production line with a textured paper surface, and that the jointing is a manual operation, either by trowel or using of setting boxes.
After sanding, the different surfaces of paper and compound are evident, and for this reason, glossy paints should be avoided where possible.
It should be emphasised that in some cases, glossy paints may be desirable for purely functional reasons, e.g. kitchens, bathrooms etc. In such cases, a less uniform appearance must be accepted.
Even when work has been carried out by even the most highly skilled tradesman, the finish can disappoint if glossy paint is applied.
Dark colours may expose many of the same limitations of surface uniformity as do glossy paints or critical lighting.
SPRAY PAINTING
Contrary to popular belief, the spray painting of plasterboard is not recommended by plasterboard manufacturers. The over-dilution of paint, poor workmanship or reluctance to back roll the freshly applied surface will result in an unsatisfactory finish. The spray method of painting may aggravate face paper napping and accentuate joint lines.
It is a fact that a good plastering job can be made to look substandard by poor painting practices.
If in spite of industry recommendations, spray painting is to be used, then a proprietary plasterboard sealer coat must be roller applied before spray painting of the top coats, thus improving the parity of texture and porosity over the substrate.
SPECIAL FINISHING OF PLASTERBOARD
There will be occasions when the standard Level 4 plasterboard finish may not produce the desired result. This may reflect choice on the part of the client or specifier, and may require additional treatment to counteract specific problems on a particular job - such as critical lighting.
SMOOTH FINISHES
Level 4 is the standard finish for plasterboard in residential and light commercial applications. Because the final surface is composed of paper faced board jointed by sanded setting compounds, the finished surface differs in texture, and porosity. Where there is application of gloss paints, dark colours, or critical or reflected lighting a Level 5 finish must be considered.
SURFACE MODIFIERS / FILLED SEALERS
There is a small range of surface treatments which combine some of the characteristics of a very thin skim coating with the ease of application of a paint. These are essentially paint carriers with sandable fillers. Your local distributor of paint products can advise you on available products and their capabilities and limitations.
In general, they offer an ability to present the painter with a more uniform surface texture but may require multiple coats to even out small variations in level. They provide an improved level 4 finish but may not achieve a full level 5 finish.
TEXTURED COATINGS
In circumstances where critical or reflected lighting could be particularly intense, it may be worth considering a textured coating system in which the texture disguises minor variations in the surface. Texture coatings have an aesthetic appeal of their own and can of course be specified purely for their visual effect.
Textured coatings include the following categories:
PAINT TEXTURES - These are specially formulated coatings applied by the painter which are worked or stippled to produce a light textured finish. Generally, paint textures have a relatively low profile or relief, but can be successful in offering a finish which is both 'different' and tolerant of minor surface irregularities.
SPRAY PLASTER TEXTURES - Spray Plaster is used as a base for proprietary finishes usually containing an aggregate forming the 'texture'.
Texture coatings applied by spray fall into two categories:
Decorative Finishes for textured applications usually containing Perlite or polystyrene.
Passive Fire applications usually contain vermiculite.
The equipment used to apply these is sophisticated and capable of varying the texture from 'fine' to 'coarse'. The texture can be further modified on the wall or ceiling surface by 'knocking down' with a trowel.
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