Seepage? No Seepage!
general journal junk
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idealog
The secret to making sure that your paint doesn't seep under your masking tape when using acrylic paints is to run a bit of clear gesso (or white, if that's the substrate colour you're using) over the spot where the tape and paper meet. Just be careful when removing the tape to also run a blade of some kind along the seam when you're pulling the tape up so that you don't pull the actual art up with the tape. Also, when you're pulling the tape up, pull away from the painting.
This works on both paper and canvas, and might have some degree of success on unprimed canvas as well, although I have not tried it yet.
This may also work with watercolour ground when working with watercolour paint as well, but I have not tried it. Running a thin line of masking fluid along the tape edge would provide some degree of success in maintaining a barrier, given that that's what masking fluid is for. But remember to be careful when removing it that you don't remove your art too.