The cap is a thin polymer shell - typically 0.3 to 1.2 mm - fused to the board's wood-plastic core during manufacturing. It doesn't make the board stronger. It makes the surface non-organic. That's the whole trick: fading, staining, and mould all need exposed wood fibre to work on, and the cap puts a plastic skin between the wood flour and the weather. Uncapped composite is the same wood-plastic mix all the way through, surface to core. The exposed fibre is why it greys, stai