Masking fluid is just one of those funky artist products I like to use. However, (I think) the outcome can sometimes be unpredictable. If you don't know what the heck I'm talking about, in a nutshell, masking fluid is a "liquid tape" that you brush on your artwork in areas you don't want painted.
Here is an example of what it I'm talking about. You can tell in some cases I'm not a perfectionist because I just slopped the mask all over the place. The white part is a flower stencil that I put masking fluid on. When it's dry it will feel like rubber and you can just rub it off. I use my finger or you can use a rubber cement pick-up (mine is the Pro Art brand).
Ok, here is the tricky part. I use a lot of vintage book, music and dictionary pages in my work. I hate to say the word "can't" but you kinda "can't" use masking fluid on those papers. They are just too delicate and when you go to rub it off it will take the paper with it.
It's best to use it with heavy weight watercolor paper or illustration board. Another quick tip: your paint, ink, whatever you use will not dry on top of the mask so make sure to clean it off with a Q-tip or towel before you start rubbing it off.
FYI: in the artwork above I used Ranger inks. They are cool beans because they are transparent but still vibrant.