Wood varnishes and wood seals

Protection of exterior wood can cause a lot of head-aches if the right route isn't followed from the outset. Often the quality of wood used in South Africa is not the best, and this only adds to the problem. If new work is being done, start off by using the best timber you can afford. Hard, dense woods are more dimensionally stable than softer woods, and the additional cost is money well spent.  Harder woods are less porous than soft woods, meaning there is less swelling due to water ingress, and ultimately less chance of rotting. They are also less prone to warping due to heat - both of these factors lead to less stress being placed on the coating, from factors other than uv light and the elements. Having installed the timber, check with the timber supplier whether the density is suitable to be coated with a varnish, or a wood-seal, as very dense timbers should only, at the most, be protected by a wood-seal.
Preparation of new timber involves sanding the surfaces to be coated to a smooth finish and removing dust. Knots (if present) should be sealed using a knot-sealer as per the manufacturer's instructions.

• Varnishes are typically solutions of long oil-length alkyds (a solvent based binder type) containing a low level of Linseed Oil ( to lubricate the upper-most wood fibres). Other additives could include u.v. absorbers and transparent iron oxide colourants. Typically this finish is glossy. The coating will have a limited penetration into the timber - so thinning the first coat up to 10% with Mineral Turps is advised. Allow this to dry for 24 hours, and thereafter apply the first of two top-coats (unthinned) with 24 hours between coats. Maintenance of these coatings would involve sanding the coated timber to an even, matt finish, and re-applying one unthinned coat every 12-18 months.
• Wood-Seals are dilute solutions of long oil-length alkyd, with a higher level of Linseed Oil, and waxes. The same additives are used as for the varnishes. There is generally more penetration of a wood-seal into the timber than varnishes, and the product has a matt, waxy appearance. They take longer to dry - due to the higher Linseed Oil level, and for this reason are seldom used indoors. Three coats can be applied (at 24 hr intervals) to the prepared timber. Maintenance of these coatings involves cleaning the surface with a rag soaked in Lacquer Thinners, allowing to dry, and recoating. Maintenance intervals are the same for each coating type.