Melamine resin treated as well as varnish coated and untreated solid wood during two years of natural weathering

Samples of solid wood, impregnated with melamine resin or coated with varni sh, as well as untreated Scots pine sapwood (Pinus sylvestris L.), Norway s pruce (Picea abies L.), English oak heartwood (Quercus robur L.) and Dougla s fir heartwood (Pseudotsuga menziesii France.) were exposed to natural wea thering without ground contact for two years. The melamine resin even in lo w concentrations provided distinctive protection against photochemical wood degradation by weathering and infestation by wood staining fungi, but did not protect against cracking and high moisture content. In the second year of natural weathering the samples exceeded 25% moisture content on 9 days ( untreated Douglas fir), 21 days (untreated oak), 88 days (untreated spruce) , 129 to 160 days (pine impregnated with MF resin), 166 days (untreated pin e) and 236 days (pine with varnish). After two years of weathering, the sam ples treated with biocide-free, transparent varnish gained a moisture conte nt of 130% within 1 hour of floating in water. Light microscopy revealed ex tensive bacterial colonisation in the interior of the varnished samples whi ch is made responsible for the increased water adsorption. The varnish, whi ch was perforated in numerous places by blue stain but nevertheless still r estricted diffusion, induced long-lasting periods of high moisture content. Thus the conditions necessary for the growth of bacteria and soft rot were provided and the risk of decay increased by the damaged coating.