How the substrate affects external timber doors

When external timber doors have been selected, it is important to note that they contain a number of substrate features that could affect their performance, either now or in the future. This article takes a closer look at some of these features and the affects they can have. Sometimes, simply painting or staining doors can help prevent any issues from occurring:

• Density – The heavier and more dense the species selected, the less prone it will be to absorbing moisture. The lighter the species, the more prone it will be to taking on moisture.
  
• Edges – The presence of sharp areas can create stress in paint films, eventually causing them to fail. Arrised or rounded edges permit film movement with minimal stress.
• Unseasoned (Green) – The use of timber that is “green” increases the likelihood of blistering and peeling in the paint. A stain will generally be more effective.
  
• Weathered vs unweathered – External timber doors that have been exposed to weathering prior to being finished often experience poor paint adhesion, but stains don’t tend to be affected.
  
• Heartwood vs Sapwood – Heartwood has better staining potential than sapwood, but both are quite suitable for being painted.
• Smooth vs coarse – Surfaces with a smooth texture are able to hold paint better than those that are coarse. Surfaces with a coarse texture, however, will provide a better performance.
  
• Gum pockets – The presence of gum pockets in the door can lead to resin exudation, unless they have been pre-treated and sealed. This is important for maintaining the appearance.
  
• Knots – The presence of knots in the timber can lead to premature cracking, staining and resin exudation. These occurrences can be avoided using knotting varnish or on manufacturer’s advice.
  
• Bark – If any pieces of bark, no matter how small, are allowed to remain on the door it could lead to the premature failure of any coating (including both paint and stains).
  
• Insect damage – Sometimes insect damage cannot be avoided. The damage must be treated or filled before finishing to avoid failure.
  
• Moisture range – The normal range is between 10 and 15%. Levels that are higher can be very detrimental, particularly if a solvent borne finish is used (where moisture vapour permeability is low).

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