Frustrated with the inconsistent results from the masking tape, unprimed canvas and watered down acrylic approach I started to think if there was an alternative technique I could use to achieve the same effect? I started thinking about dry pastels as they would not have the same issues with running and they should be able to produce a nice clean line when used with masking tape.
I used some unprimed canvas to experiment with the pastels. As can be seen in the picture to the left the pastels gave a nice clean edge when used with the masking tape. However they just did not look as aesthetically pleasing as the earlier painted canvas. So this was a solution as such but not an adequate one.
At this stage I also started thinking about using a size on the canvas such as rabbit skin glue as this would effectively seal the canvas and prevent the paint soaking in to it too much.
I purchased some rabbit skin glue and sized a canvas. After waiting for it to dry I then masked up some square areas and painted them with the watered down acrylics. Although the sized canvas was more water resistant than the raw unprimed canvas it still did not stop the paint seeping underneath the masking tape.