Jeff Ishaq wrote:
Hello,

I'm painting a square room, and one wall is to be red; the other walls
should be white. All the walls are spackled. I am having trouble
getting a perfectly straight vertical line in the very corner of the
room, where the red paint of one wall ends, and the white paint of
another wall begins. The slight protrusion of the spackle makes this
transition look very jagged, as it also causes paint to bleed
underneath the masking tape when I try to mask off a nice line.

Is there a trick to getting a nice straight vertical line that
transitions from one color to the other in this situation? Someone
suggested to me 'gum arabic', but I haven't the slightest idea what it
is, or what I might do with it.

Thanks!
-Jeff

It would be unusual for any corner to be perfectly straight. Paint the
white walls into the corner. Let dry at least two days. Get a plumb
bob and chalk line and snap a straight line at each corner so the the
corners are in the red area. The line should be as close to the corner
as possible. Put down painters tape on the white, paint your red wall
and remove the tape before the paint dries. If you press the tape on
carefully, you should not get any bleeding under the tape edge.