Here is an idea to consider for treenails at 1/75 scale…take a pointed tool and gently dimple the planking at the point you want a treenail. Not too deep…just a hint of a dimple or depression. Then, when your deck planking is finished, lay down a coating of semigloss (or matte) polyurethane. The poly will settle into the dimples and the plank edges to give the impression of treenails and edge caulking both without overwhelming the scale. The slight coloring of the poly as it settles in the dimples gives the perception that a tree nail was placed. This is all about perception. Scale is very difficult to maintain at 1/75…most everything we try to do is too big for scale!