Hi ya geeg, hope you are well and looking forward to Christmas.
Yep, cotton thread! As you follow the line with your brush, the varnish dips into the dent and you get a perfect line. You can only do straight line with this. If you have my book, there is a full page step by step pic. It sounds a bit mad but its good confidence building.
I still get it on the skin, but a quick wipe and its gone. i would rather have a perfect line.
I have also started using the Nubar 2-way nail art pen to draw the line, then a varnish to fill in the bottom.