First and foremost youll have to get the resin surface clean and somewhat rough enough for anything to stick to it. Ive no idea how the surface of your casting is but if its just plain resin with the general smooth yet shiny surface, take some 320 grit or higher sandpaper and give it a once over. This will give the paint/material something to adhere to. The grit may be closer to 500/600 that you need but unless I had the item in my hands I couldnt tell you. Resin has an inherent issue of underlying air bubbles just below the surface. If you scratch the surface too much, you may well end up with tons of tiny holes everywhere.
I use a combo of off white/tan latex paint and pros-aide medical adhesive as a base. Its the easiest yet most effective and simple way to do wood grain. Trick is, as mentioned above, layers. Easiest way is to have a dark base (such as black rubber which is what many film props are made from) then apply the paint/adhesive mix with a chip brush creating the grain effect. It matters not if the item in question has cast in wood grain, its the color were after.
A heat gun run over the surface will dry it in seconds.
Then its a matter of choosing the proper colors, and this is the other key element, of Mohawk spray on wood toner. Its not available worldwide and no I dont have another brand to recommend nor how other brands may perform. I only use Mohawk. In very light mists spray it on allowing each layer to dry inbetween. This takes seconds again with a heat quickly run over the surface. Once you get the hang of the color layers it will take a minute or two to do wood grain from start to finish. Once the color is done I sometimes use a matte or oil finish and dust the surface. It gives a break up and worn look to the finish. Krylon flat black also does this but thats a real learning curve to master. That paint will eat into the finish and ruin it with one bad spray.
Some examples. The woodgrain itself on everything below took minutes to complete.
A plastic airsoft rifle that started out all black.
Rubber Mauser grip:
Airbrushed over resin Mal pistol:
and of course a super quick and dirty rubber Jawa blaster done for EP3 Burger King commercials:
