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 and guidance on the new Parts L and F of the building regulations. This week: What Causes The Discolouration Of Paint Finishes Over Knots And How Can This Be Treated? Resin is produced in a growing tree at points where the tree has been injured. For example, resin pockets can form where there has been frost damage to the layer directly under the bark, or where a knot has not fully inter-grown with the surrounding timber and resin is deposited in the gap and in the knot itself. The resin pockets will be covered over as the tree grows and can be uncovered when the timber is sawn or moulded. No new resin will be formed once the tree is sawn into timber and if left long enough the resin will harden by evaporation. However, if the timber has not been left long enough before use, or the resin is softened by heat, it can run from the timber causing non-porous finishes to blister. Furthermore, if the finish is porous, the resin can pass through the finish and appear as runs or crystals on the surface. Where resin has passed though a porous finish and the coating is not damaged the resin is best left to dry and harden when it will easier to remove by scraping or brushing. To speed up the process, heat can be applied to the affected area, which will encourage the resin to flow, and the surface resin can be removed using a cloth moistened with methylated spirit. This process can similarly be used to prepare the surface where resin has lifted a non-porous finish prior to repairing the damaged coating. If there is a darkening of the surface finish over knots (particularly associated with European Redwood and Douglas fir) it is a result of the extractives – a blend of resin acids or rosin in turpentine, with small amounts of tannins, which are water soluble and darken on exposure to light. These extractives can migrate into the surface finish to produce a brown discolouration in the area of paint over the knot. The effects of extractives, which tend to be more prevalent with waterborne paints, can be reduced by the use of stain-blocking primers or primers with reactive pigments. Traditional knotting agents, such as shellac, can be brittle and do not adhere to the base timber as well as primers and can crack or lift when attempting to block the extractives or resins.