I do quite a bit of resin filling, the resin I use is as thin as water until it sets and I have no trouble getting it in to the finest detail, I apply my resin using a dropper and a syringe for very small detail. I work mainly on glass and stone but it works equally well for metal, Can be baked in an oven as well for fast curing.
The resin is 2 part and i can adjust the opacity and colour using colourants, also get some very good effects using colours as you would a water painting.