Woodstains are formulated to become thinner and thinner as they weather, which makes them easy to maintain, but if applied too thinly it will mean that more frequent maintenance will be required.
They must be thoroughly removed from the surface prior to decoration, and this can be effected by thoroughly rubbing the timber surface with a lint free cloth dampened with cellulose thinners or methylated spirits, changing the face of the cloth regularly. Although Iroko is not oily, it contains natural chemicals which have a similar effect to the oils in teak, and the surface should again be thoroughly degreased before coatings are applied.
High solids coatings such as Supercoat and One Coat are more susceptible to this problem than traditional coatings.
These oils migrate into the timber and oxidise with the atmosphere unless the compound is sealed by a film-forming system, such as a traditional gloss paint. Sadolin woodstains are moisture vapour permeable, and therefore do not seal the glazing compound. The resulting loss of the oils allows the glazing compound to dry and crack, causing a failure of the glazing seal.
For this reason we recommend the use of approved elastomeric sealants (polysulphide, silicone or acrylic).
Under the influence of atmospheric moisture, it can migrate to the plywood surface and appears as a white crystalline powder. In the normal course of events, the action of rainwater washes these salts away and they are rarely visible. However, in sheltered areas, such as soffits, the salts are not removed by rain action and remain as a ‘bloom’ on the surface.
The remedy for this is to wash down the affected areas with clean water and a bristle brush (not metallic bristles), thereby removing the salts. Provided that the finish is not damaged by over-vigorous scrubbing it need not be re-treated. It may, at some time in the future, reoccur to a lesser extent before the salts are fully depleted. Again, removal is by washing.
This phenomenon seems to vary with the type of plywood and country of origin. It should be pointed out that the loss of these salts in no way affects the strength or integrity of the plywood and does not blister or crack the finish.
This process takes as long as 12 hours, and it is therefore essential that the coating is left to dry overnight before overcoating. Applying the second coat too soon can lead to wrinkling and a soft finish which can be easily damaged.