Surface treatment of wood and metal outdoors

Outdoor wood and metal are exposed to a different kind of stress than masonry facades. Wood works with moisture fluctuations and can rot if not properly protected, and untreated steel rusts when exposed to moist air. As a building painter much of the outdoor work is therefore concentrated on windows, doors, boards, and metal parts.

§Woodwork — linseed oil and modern alternatives

Linseed oil paint has historically been a cornerstone in the treatment of external timber because the oil penetrates the wood's fibers and gives a flexible, breathable surface that moves with the wood instead of cracking. Many old windows and doors are still protected by linseed oil paint. Today there are also modern, faster-drying alkyd and water-based products for exterior timber, which may be easier to work with, but have different requirements for substrate moisture content and surface preparation.

§Assessment of substrate before treatment

Before you paint exterior woodwork, you must assess whether the wood is sound, if there is rot or fungus, and whether the moisture content is low enough for the paint to adhere and cure properly. Painting over wet or damaged wood is wasted work — the paint will quickly crack, or the damage will continue under the new surface without being visible.

• 01Check moisture content in wood, preferably with a moisture meter
• 02Look for signs of rot fungus or insect attack
• 03Sand or scrape loose old paint down to solid base
• 04Base bare wood before top paint is applied

§Metal — rust is the enemy

Untreated or damaged metal rusts in contact with moisture and oxygen, and rust under a paint film spreads further, even if the surface looks fine from the outside. Therefore, treatment of metal always starts by removing loose rust and old, poorly adhering paint — typically by grinding or brushing — before a rust-protective primer is applied. The primer is the part of the system that actually protects against new rust formation; the topcoat primarily provides colour and some mechanical and weather protection on top.

§Systems thinking

Both wood and metal are best treated as a system of products designed together — primer, intermediate coat and topcoat must be compatible and from the same or recommended product series. Mixing random products from different systems can result in poor adhesion or chemical reactions between layers. The manufacturer's instructions for the complete system are therefore a better starting point than choosing individual products on price alone.

Outdoor wood and metal work is a good example of the fact that the core of the painter's trade—assess the substrate choose the right system prepare thoroughly—applies just as much here as with facades and indoor tasks.