I use the one for watercolor. I use it just for masking off the eyes (or the Kirin’s horn) and I apply it thick (mine has thickened over time so it is chunker than the new watery form)and the acrylic hasn’t penetrated it.
I use the masking fluid because I spray prime and then spray seal the final piece and the eyes NEED to be covered in this process.
I have used masking tape as well. I actually find this pretty useful and inexpensive. Also, I haven’t tried the masking fluid when it is in it’s watery form, so that might be a pain to work with.