There are ''Rigging Brushes'' which are exceedingly fine long bristled brushes, or there's even a ''Trainard'', which is a brush with a single hair!!
But there's also a technique that was used in oil painting by marine artists in the past where the rigging lines would be painted in, then a layer of varnish was applied. Left overnight, the varnish shrank the width of the painted lines of the rigging till they became exceptionally fine on the canvas. Many years ago I saw a TV program that demonstrated this, but I've forgotten which programme, and I can't find any reference to it.