Oh, you're trying to use a grid on canvas... yeah, that's not going to erase easily. If you're using an opaque medium like acrylics, it doesn't matter so much. I paint over pencil lines on occasion. But if you're using a transparent medium on canvas you probably want to do your drawing on paper and then transfer it to canvas using a projector, carbon paper, etc.
I was just trying out what you said about trying to imagine a grid. Right now, I'm using it on a photo reference so idk how it'll work while trying to draw something from what you see. Anyway, yea it's working so far. I didn't think it'd work! haha
Imagining the grid would be difficult and very unreliable.
To OP: If you're really still seeing marks and there is no way to get rid of them. Well, try to tape some tracing paper on top of your canvas with a grid drawn over it. Then just flip it back and forth when you are in need of it. Although, this is pretty frusturating, if you're relying on the grid too much.
Besides using the grid you can try to use other duplicating techniques like sight measuring. Basically you extend your hand with a ruler or your fingers and go back and forth. Here's a good read on how to do it - [link]
That's all I can come up with right now, hope you will find this useful in any way.
Using the tracing paper does seem a bit frustrating but I'll give it a try sometime :3
And I definately have heard of sight measuring but never really paid attention to how I should do it so thanks for the link! I'll have to give that a try as well
Also, I was thinking of finding a way to use a light under the paper. My dad has this big squre light that was used for looking at x-rays. I might be able to use that to see through the paper and under the paper I would put a grid so I could see the grid through the paper. It's, again, something I'll try sometime.