BWF members such as yourselves frequently use our Member and Technical Helplines for assistance on a wide range of Employment, Health and Safety, Pay and Technical issues. We’re putting up a frequently asked question every Tuesday in order to help members and illustrate the type of advice we are able to provide. If you have any questions that you would like to see featured, we would be happy to assist. This question and many others like it can be found in our Question Centre. If you want further advice on any of the issues, don’t hesitate to ring the helpline or browse our extensive publications library, which includes our advice on resin exudation in joinery timber, care of products on site, and guidance on the new Parts L and F of the building regulations. This week: How Can I Ensure Longevity In A Paint Finish? In a recent issue of Join In No. 20 – 15th October 2010, Our Technical Manager, Kevin Underwood, discussed the use of blocking primers and knotting agents to inhibit the effects of extractives from timber. However, it is not just the exuded resins, tannins and gums that need to be considered to ensure the long-term performance of a finishing system. Thought needs to be given to the design of the joints and profiles, the condition of the timber, the finishing process and the finishing system employed. BS 644, the standard for timber windows, contains a lot of requirements for the finishing of windows, requirements which can equally be applied to doors. The profiles and joints should be designed to avoid trapping water by utilising sloping surfaces or drainage channels, and by consideration to capillary effects where water is drawn into a thin gap between two components. Any sharp arrises should be softened with a 3mm radius to prevent the paint film being thinned as it passes over the edge. The finishing clause in BS 644, which has recently been reviewed by our window and door technical committees, requires finishing systems to be assessed to BS EN 927 for stable end use. This means that the finishing system will assist in controlling movement due to changes in moisture content. The clause also requires the finishing system to inhibit mould growth and blue stain and there are requirements for a minimum dry film thickness. To allow the finishing system to perform at its best it needs to be applied correctly in accordance with the manufacturer’s recommendations. It is important to start the process by storing the finishing materials in appropriate conditions and ensuring that their shelf life has not been exceeded. The timber should be prepared so that its surface is suitable to receive the finish, and it should have an appropriate moisture content. Some painting machines utilise electrostatic attraction to draw the paint on to the surfaces and this does not function if the moisture content is too low. Similarly, the surfaces of the timber should be at an appropriate temperature and not too wet. This is a particular consideration if the finishing system is to be applied on site rather than in a workshop. The members of the BWF Timber Window Accreditation Scheme, which incorporates the BSI Kitemark to BS 644, will have their finishing processes documented as part of their factory production control procedures. Documenting the manufacturing processes is good practice and can be employed by non-accredited window and door producers too. Working with the paint supplier, good practices for the finishing processes can be developed into a series of instructions which can also describe the checks required to ensure that the processes have been done correctly.