I would follow the advice of sticking with the thick one and step over the detail areas.
I use these thin pens but I find they are too hard to see on the chart regardless of the colour I chose. But if you want thin these do the job. And your sweaty hand doesn't smudge them off.
http://www.transair.co.uk/sp+Lumocolour-Chart-Pen-Pack+4660
Good luck with the cpl.