In order to fully protect the timber substrate an exterior timber finish must be able to perform the following functions:

1. Maintain film flexibility for the life of the coating to resist grain cracking and aid adhesion.
2. Resist degrading from ultra violet light.
3. Allow moisture vapour transmission to prevent blistering from entrapped moisture.
4. Impart water repellency to maintain less than 20% moisture content in order to prevent fungal attack causing dry rot.
5. Resist fungal attack in the form of unsightly and damaging organic growth, mould and mildew.
6. Weather by gradual surface erosion to facilitate ease of recoating.

The more pigment in the product the more the product will stand up to UV break down. Paints contain more pigment than stains.