Most of the problems people have with painting involve getting a nice
straight line when cutting in, and most of the time the reason they're
having problems is because they're not using a sash brush properly.
Here's a typical sash brush:
People look at a brush like that and think that the bristles are cut
longer on one side so that the brush will fit better into corners, like
and that's where they go wrong and end up making a mess with their
The reason why the bristles are cut shorter on one side of a sash brush
is so that all the bristles flare out the same amount when the sash
brush is used properly, like this:
That is, when you use a sash brush properly, it's the shorter bristles
that are at the "leading edge" of the brush and the longer bristles that
are at the trailing end of the brush. By cutting the bristles shorter
on the leading edge of the brush, all the bristles "flare out" the same
amount when you use the brush properly, and that allows you to get a
nice sharp edge when cutting in.
Try using your sash brush as described above, and see if that helps.
PS: Compare the results. The guy who's using his sash brush wrong is
getting all kinds of yellow paint on his blue masking tape. The guy
who's using his sash brush correctly doesn't even need to use masking
tape. People often get poor results simply because they're not using
their tools correctly.