This product may have been around for a while now, but I have only
just heard about it. Working with watercolour you invariably get to work
with masking fluid (or 'frisket'). Larger areas are not a problem, I
have old brushes reserved for the job, but over the years I have tried
lots of methods with varying degrees of success and frustration to get
clean, fine lines. I have heard of people using an 'oiler boiler' -
perhaps this is a similar idea? Anyway, after posting details of how I
paint light hair over dark (I use opaque paint), I was told about the
masquepen and had to give it a go. I thought it might be of interest to
others to hear how I got on with it and have included some trial doodles
below:

I ordered a regular pen and a 'superfine' nib as well (shown at left
on the picture above). It certainly pays to experiment with it
before launching into a painting. It takes a little while to get
used to having a bottle attached to the pen, but the applicator is
long enough for you to hold comfortably and draw. The fluid is
thinner than normal masking fluid (and blue) - more like the
consistency of thick ink. As the flow is dictated by gravity, you
need to adjust the angle and speed that you work or you will get
'globs'. The regular nib is shown at the top of this doodle, and the
superfine nib at the bottom.

It is quite fun to work with, and I can see how it would be useful
for craftwork too. The results were pretty good and there is no
question it will be helpful for things like whiskers, leaf veins,
grasses etc - infact anywhere that you need a fairly precise thin
line that you can control. Of course if it didn't go on right, you
can just wait for it to dry and remove it in the usual way.
I think it will pay to mess about with it some more, but my first
impression is very good and I will be using it again. Whether you
need the superfine nib as well will depend on how fine you want your
lines. It does what it says, no mess and no fuss - and no more
ruined brushes.