This is my favorite detail brush. It goes to a very fine tip and is perfect for those last tiny details that can be the most difficult. I believe this brush is synthetic but it holds it's tip well if you take good care of it. I am currantly out of stock but you can get it at Dick Blick: https://www.dickblick.com/products/princeton-velvetouch-synthetic-mini-brushes/?item=06432-1200&
20/0 Liner Art Brush
$9.00Price