A finish will only have a long life if it is suitable for its location. Barbour Index and Scott Brownrigg look at materials and how they stand up to various environmental conditions

Material finishes have a difficult job. A thin layer has to protect the material from a host of aggressive external conditions, and look good too. It also has to last as long as possible before it is replaced. Finishes may be applied to a substrate or can be an adaptation of a material’s natural surface. This is fundamental and must be determined when considering the application.

The key thing for the specifier to consider is the environment where the finish is being used. In any location that has harsh parameters combined with a significant life requirement you will to careful to select the right finish.

1 - External finishes

Externally, any finish is subject to the effects of weathering. This includes moisture in the form of driven rain, snow and frost, and can weaken joints in particular. Consider the potentially damaging effects of freeze-thaw cycles on ceramic or masonry surfaces and ensure the material and especially any glaze has the ability to withstand this. Solar radiation breaks down dyes, paints, and adhesives at molecular level. This manifests itself as de-bonding, flaking of paint and the fading of colours; bright reds are the worst affected. These effects are amplified in a marine environment, which in practical terms is within five miles of the sea. Marine environments experience the damaging effects of salt, wind-driven sand and stronger ultra-violet radiation.

As a rule, if even a moderate life expectancy is required, only natural finishes should be used. Any applied finish will age rapidly without significant care and maintenance. Salt can also accelerate effects normally not considered a problem. Dissimilar materials must be considered and any potential electrolytic combinations avoided. Specifications should also ensure that any cut edges are correctly addressed to avoid premature corrosion. In extreme locations, glass can become milky due to sand abrasion.

2 - Interior finishes

Interior finishes must be chosen to suit the location and the kind of wear they will endure. Areas that are likely to receive high use, such as entrance hall floors, common handrails and main access door ironmongery will wear quickly unless the correct standard of finish is specified. Most finishes have an appropriate trade standard allowing the specifier to match the requirements to the quality. A good starting point is to review the trade body advice, which is generally available online.

Consider the life expectancy, which can be divided into three phases. The first is the period over which the manufacturer will correct any defects – usually through the warranty or guarantee. The second phase is the period to the first set of significant maintenance. The final phase is the time until the finish needs replacing. Reliable data on life expectancy can be difficult to obtain. All specifiers should ask for definitive evidence of lifetime performance. Check installed samples at five- and 10-year intervals, as time spent reviewing built examples can be invaluable. Enforced ageing is not always reliable but if no other data is available it is better than desk-based data.

3 - Appearance

The specifier needs to be clear about the aesthetic requirements and how this translates into a technical specification. Ensure you understand the limits and tolerances and are familiar with industry standards.

Natural finishes should be specified with levels of surface texture, be it rough or smooth. Applied finishes can be controlled in terms of thickness and method of application. It is wise to always have a quality control sample that is agreed with the whole team from the outset. Polyester powder coatings can be controlled within a few microns but should be prescribed to suit the location, normally 60-70 microns for external applications. If the coating is any thicker, the result will be an orange peel finish.

Always obtain a sample from the actual production line providing the project’s polyester powder coating as quality can vary considerably. Anodising is a dye process that chemically bonds with the surface of the metal. Although techniques have improved in recent years, high and low limits should always be called for prior to manufacture.

Metal finishes that are required to weather naturally must also be considered with care. Any pre-existing contamination may manifest itself as the metal oxidises and variation across large areas is sometimes unwelcome. Again, site visits are more than worth the time expended.

4 - Compatibility

Compatibility is a significant issue with finishes. The specifier must always consider the substrate under the finish, plus the adjoining finishes. Always ensure the correct finish is used for the specific substrate and environment. Moisture or the migration of solvents within the finish can quickly cause degradation to any vulnerable surface. Include bond breakers behind seals and moisture control in wall and roof sections to ensure the seal bonds properly to the joint. Some sealants can Leach into absorbent finishes – always check with the manufacturer.

5 - Repair and maintenance

Consider how repairs may be undertaken, as applied finishes can be repaired almost seamlessly if done expertly. Natural finishes are more weather resistant but harder to repair. Absorbent finishes such as stone are difficult to clean. Any vandalism needs to be repaired by a specialist.

Regulations, in particular the Disability Discrimination Act, require the specifier to consider finishes from the perspective of the user. Contrast is a key issue and guidance recently published by the Guild of Architectural Ironmongers sets out to clarify this. Further help is also available from a guide, Colour Contrast and Perception, published by the University of Reading. Information is available on the website of the university’s inclusive environments group, www.rdg.ac.uk/ie.

Technology is coming to the aid of the specifier. More robust finishes continue to arrive, but there are risks. In the 1970s polyurethane paints were specified for exterior woodwork. These were very tough but prevented the wood from breathing, causing premature rotting. Be clear about the application and the chemistry involved.


  • Technical Update March 2005, Guild of Architectural Ironmongers

  • Colour Contrast & Perception, The Research Group for Inclusive Environments, Reading University 2004

Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification Expert services. For further information, contact Barbour Index on 01344-899280 or visit www.barbour-index.co.uk

Paints, sealants and finishes