Finishing and decoration of Plasterboard
A joint Industry approach from NZ’s Association of Wall and Ceiling Industries (AWCI) and the Australian Paint Manufacturer’s Federation, recognising the need for a clear and concise position on the finishing and decoration of plasterboard, have collaborated on the publication of this Information Sheet.
This document identifies the standards of finish which may be expected from different installation, finishing and decoration practices and problems that may arise from inappropriate choices.
A plasterboard wall or ceiling presents the painter with a surface comprising two materials of differing texture and porosity. To ensure a satisfactory finish, these differences in surface characteristics must be addressed. Unless otherwise specified a Level 4 plasterboard finish as defined in AS/ NZS Standard 2589 Gypsum linings Application and Finishing shall be taken as the standard finish for plasterboard presented for painting.
Level 4
Plasterboard should be installed on walls horizontally. Ceiling sheets should be installed parallel to primary light source and where appropriate back-blocked. Where possible full sheets should be used thus minimising the need for butt joints. Jointing should comprise a 3-coat system. Each coat should be fully dried. The top coat should be sanded to a smooth, even finish. Edges should be feathered in order to minimise scuffing of the paper face. Similarly, internal and external corners, fixings and cornice installation should be finished to the same standard. This will deliver a substrate in a suitable condition to accept paint.
Gloss Banding/Sheen Staining
These terms are used to describe the phenomena where the plasterboard joints exhibit variability in gloss level compared to the overall surface appearance. This is often due to the failure to use an appropriate sealer.
Relevance of AS2311
One of the objectives of the paint system, as defined in Australian Standard AS 2311 – Guide to the Painting of Buildings is to deliver the appearance of a uniform surface texture and colour.
Sealing the Surfaces
An essential first step is to seal the face of the plasterboard and the plaster joints with a good quality sealer undercoat. This will ensure an even “suction rate” and provide a degree of opacity for subsequent paint coats. In some cases, the sealer undercoat could be considered as the most important component within the paint system. The application of the sealer undercoat should be carried out in such a way as to ensure that the plasterboard paper face fibres remain flat. In some circumstances an initial coat of a suitable sealer will be required on surfaces subject to yellowing, bleed or sheen staining. Where that sealer is not a sealer undercoat and does not exhibit the properties of an undercoat, the sealer coat needs to be followed by an appropriate undercoat prior to the top finishing coats. Once the sealer undercoat has fully dried two top coats of water-based paint must be applied (ensuring adequate drying between coats) as defined in AS 2311. The performance of the finished paint system and the appearance of walls and ceilings are highly dependent on the quality of the paint used, application method, colour and sheen level.
Expectations of Painted Plasterboard
A level surface with no visible joints when viewed from the normal viewing position and under normal lighting conditions. • A serviceable and even sheen decorative finish. These expectations will be difficult to achieve due to: • Poor design concepts; • Poor workmanship; • Poor quality paint; • Failure to use a suitable sealer/ undercoat; • Glancing light due to natural and/or artificial lighting conditions; • Crowned or starved joints; • Insufficient drying times; • Dark coloured paints;
Gloss paints. Brush, Spray or Roll?
Brush Provides a reasonably smooth finish but slow application – not common practice. Brush application does not impart an appropriate texture.
Roller
Provides a low to a medium build ‘orange peel’ texture finish. It is generally agreed that roller application of paint is the preferred method as a uniform finish and texture is imparted to the surface with each coat of paint. Roller application also provides an even film build over the plasterboard and jointed areas.
Spray
Provides the smoothest finish and is 20% quicker than rolling. It is however difficult to touch up after the job is completed and does not deliver a uniform finish. However, if spay application is used an acceptable finish can only be achieved if the coats are “back rolled” while the paint is still wet. This will provide a uniform texture over the entire surface and flatten any raised nap or paper fibres, promote adhesion of each coat and give a uniform surface when patch repairing. Backrolling is required on level four or better surfaces.
Other problems that may arise from spray application: •
Temperature conditions may prematurely dry the paint before back rolling can impart the required texture to the surface
Over dilution of the paint will reduce paint opacity and texture
Heavy application/overloading of a single coat will soften the top coat of jointing compound resulting in a general break down of the compound
Shrink-back of topping compound around fastener heads may occur
Touch Up and Repairs
When patching or repairing gypsum plasterboard where compound is used for the repair, recoating will require full surface treatment including sealer undercoat followed by top coats.
Glancing Light
This phenomenon is also known as “Critical Lighting” and is defined as natural or artificial light projected across a surface at a low incidence angle. See example below of a painted plasterboard wall photographed over a 40 minute interval.
Which Level of Finish/Paint System?
Best
Level 5 plasterboard installation; • Sand, dust and wipe surface with damp cloth; • 1 coat of water-/oil-based sealer/ undercoat; • 2 top coats of premium paint of selected sheen level; • Sanding between all coats.
Acceptable (AS 2311 Requirement)
Level 4 plasterboard installation; Sand & remove dust; • 1 coat of acrylic sealer/ undercoat; • 2 top coats of selected flat or low sheen paint.
Not Acceptable
Sand & dust; • 1 coat of acrylic sealer/ undercoat – tinted to the final colour; • 1 top coat of flat or low sheen paint. or • Sand & dust (sometimes); • 2 coats of flat or low sheen paint. Both the above do not meet AS/ NZS standard or Plasterboard Manufacturers’ requirements and may fail to meet customer expectations by resulting in the problems outlined in this Information Sheet.
Experience has shown that nonspecified products and systems, ie, 2-coat paint systems, generally do not deliver an acceptable paint finish. In these circumstances the cause of the failure will have to be determined and appropriate rectification carried out which may be expensive and time consuming.
Summary
Whatever the system, the objective is to equalise the texture difference and suction rate of the paper face and stopped joint. This can only be achieved by good plasterboard installation practice, selection of quality paints and suitable application methods resulting in a minimum finished dry film thickness of 75 micron.
The final result can be influenced by other factors such as inappropriate lighting. These factors should be considered at the design stage. Unless the plasterboard and paint systems have been installed and/or applied strictly in accordance with the manufacturers’ specifications and those of the applicable Australian Standards, the plasterboard and paint manufacturers cannot warrant the performance and integrity of the plasterboard, the jointing and the paint systems.
Trade Hand Over Procedures
One of the issues that the AWCI has been working on for some time now is to improve the expectations and handover process between finishing trades. As professional tradespeople, I know that we address any issues with our own work and rectify promptly with the builder to a satisfactory level for the customer.
All too often, however, we are faced with call backs and disputes where the problem might be due to other trades before or after our work. To help with the management of these overlaps, the AWCI has prepared a sign off documents for your use to facilitate the handover of a “plaster ready background” before you start your work, including a pre-paint sign off prior handing over to the painter/ decorator.
All AWCI members have been emailed these documents. Please contact [email protected] if you require another copy. I recommend that these documents are used by all members and encourage them to forward copies to their builders for information prior to commencing a job.