White Gel Pen recomendations/experience

i found using some gouache on a dip pen works well for making fine lines on watercolor, with the added plus that your line can be any color you want, not just white. you have to keep cleaning your nib though, because the gouache will dry and plug up the nib. just takes a quick wipe off and away you go again! i don't dip the nib in gouache. i just use an old paint brush to apply it onto the nib. quick and easy! :)
I use the Uni-ball Signo UM-153 for thin lines over my watercolors. You just have to make sure the watercolor is really dry.