Dave, I used grey card, from Rennies top of Bold street, very cheap! I laid a plank, stuck it with alphatic (not sure on spelling?) wood glue (they sell that also!) as it works very well and dries clear, then used bostic to glue a strip of card, then the next plank, and so on, very time consuming! The card does not need to be flush at this time, you sand it down, then use ronseal clear resin so the finish does not turn into something resembling toffee
I tried to replicate teh weathered effect using a grey wash, but the plank effect was lost, so rubbed it all down and just used the stain, which ended up real nice!
look how the wood has weathered in this shot