I had always struggled getting the rigging of an old sailing ship to look halfway realistic. Usually the lines either turned out too thick or disappeared during sanding and leveling.
Eventually I completed a picture without any rigging. After mounting, leveling, and sanding, I applied two coats of polyurethane finish. After that dried, I applied clear plastic shipping tape over the areas where the rigging was located. Working from the pattern, I used a knife, ruler, and French curve to cut the lines through the tape and finish and into the wood.
Next I rubbed a mixture of white PVA glue and charcoal dust into the incised lines. After that cured overnight, I peeled off the packing tape and found the fine lines I had been seeking. I applied two more coats of polyurethane and rubbed it to an acceptable sheen. The rigging looked much better than any I had done before.