Exterior varnishes, even those with U V (Ultraviolet) absorbers must be recoated more frequently than paints. For maximum durability, a minimum of five coats should be applied to a new wood surface and annual recoating with one or two coats of varnish is recommended. The best time to recoat exterior varnish is before it looks like it needs it. If varnished wood is allowed to weather to the point that the finish has gone flat and some discolouration has occurred in the wood, complete stripping, sanding, and refinishing will be necessary.
Exterior Varnish Over Stain
It is not advisable to varnish over large exterior stained surfaces such as siding and decks. Although the finish may be attractive initially, the varnish must be recoated after a maximum of two years exposure. If recoating of the varnish is not done before it starts showing signs of breakdown, stripping will be necessary, which will remove the varnish and some of the stain. The porosity of the wood has now been effected by penetration of resin from the stain and varnish, so getting a uniform appearance with a transparent or semitransparent stain is very difficult. Uneven sheen (flashing) of solid hide stains can also result.
Alkyd House Paints
Alkyd house paints are alkyds modified with either linseed or soybean oil to make them more flexible for exterior use. This modification makes them too soft to be used as a porch paint, and if used inside they tend to yellow much more than a regular alkyd enamel.
Chalking is part of the aging process of paint. The binder (resin) breaks down with exposure to the sun, leaving unbound pigment (chalk) on the surface. Repainting should be done before the chalking becomes excessive. In the early stages of surface chalking (in most cases five years' exposure), the film integrity is still good and all that is usually necessary before repainting, is to wash off the surface dirt and chalk.
Good housekeeping practices are as important in the control of mildew growth as the addition of fungicides to the paint. A film or surface residue of dust or dirt can insulate the mildew spores from the fungicide and make it ineffective. Exterior painted surfaces should be washed periodically, especially in areas of the country where mildew is a problem.
Dark colors can cause wood surfaces to become very hot. In the extreme, blacks and dark browns can cause cracking and/or cupping of wood in direct exposure to summer sun. Dark colors can also cause natural resins or sap in the wood to migrate or be drawn out, which can in turn cause paint to peel.