Hope this may help some ...when i penciled in the planking effect...it looked a bit bland so i decided to use some acrylic dark brown and thin it with water to the consistancy of a water colour....and brushed it on with a 1/2 inch artist fitch brush..(thet are quite thin)...let it dry and then using the brush sideways(the thin side)..applied a second coat to various planks to darken them up...when dry ...give it a light coat of acrylic matt varnish ...since you do not see much of it... it's a cost effective way of having a good planking effect...

.John...