How to prevent drawing over existing pixels on a Canvas Render Target?

Development Rendering
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1

Hello!

Question 1:
I m using a Canvas Render Target and drawing a material ‘brush’ onto it. Is there a way to prevent drawing where I ve already drawn?

Question 2:
I want to create a lawn and mow it using a Render Target. So I ll have some irregular plane shapes (image below), and I want to calculate how much of the surface is covered by a brush so I know when I ve finished mowing the entire plane.
I m not sure if this is possible with shapes like these tho..

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I don t have much experience with Render Targets, so if you know of a tutorial or could show me the steps, I d really appreciate it.