your local hobby shop will sell microballons, it looks like talcum powder, in fact talcom powder might work. The microballons are a lot more finer (bad grammer, but you get the point). When mixed with epoxy (two part) it makes a real paste that is not runny. Then you smooth the mixture into the cracks. It sets up pretty hard, yet, not too hard to sand. Use sparingly, but get it into those fissures.