Erasing Non-Photo Blue Pencil

Uh, non photo blue pencil isn't really designed to erase. Watercolor paper is often meant to be stretched while damp and then dried, and the texture of the paper is kind of important. You'll destroy that trying to rub out oily pencil lines. Either A. *Lightly* sketch on the paper with a hard graphite pencil so it stays like a 2H or a 4H (don't press hard. Seriously. Light) Or b. Use a softer, dark pencil (maybe a hb or a 2b) and draw a mirror image on the back of the paper, and then paint on top of a light box. The problem with this option is you can't streach the paper.

My mother is a professional watercolor illustrator. She says, "Prismacolor doesn't erase from WC paper or print paper because the pencil is slightly waxy, and the paper is soft enough that it gets etched by the point. Better to use a graphite or pastel pencil that can be erased with a kneaded eraser and use a very light touch."

It doesn't always erase. You can try an electric eraser; they vibrate at a high speed and are sometimes more gentle. Also try to "Moo" brand white polymer erasers. If you are working digitally, the lines can be removed as part of scanning. If you're not, my usual solution is to go in with a white masking liquid where I need to cover stubborn lines, then paint over that. When I was designing for black and white high-contrast Xeroxing, I also just cut out the places I wanted to be white and re-pasted before going to copy. If you can't erase, cover or remove.

You need to transfer your drawing onto the watercolor paper after first drawing it on something else. You could use a lightbox for this, use carbon & transfer paper, or you could get a printer that won't bend your watercolor paper, digitally scan & clean up & print that onto the watercolor paper, although you need to make sure your printer ink of choice will play nicely with your watercolors. You can get lots of really interesting effects this way. Mostly, though, do your blues on another thin paper, dark linework on top of that to refine, then transfer that onto your final paper. No blue on the watercolor paper. It's too fragile.

Prismacolor makes a non photo blue pencil that is presumably meant to be erasable, coming with an eraser on the end of it and all. You could try that, but it doesn't address the issues of watercolour paper raised by the posters above.

Another method is to do your penciling (on any paper, in regular pencil), scan them in, and print out onto watercolor paper, in very very light grey or blue. Depending on your end style (how precise your penciling is, how heavy/wide your inking is, if you're doing a light wash of watercolors or a full multilayer painting, if you're able/willing to digitally clean up the pencils, etc) this might not suit you, but I know that at least 2 of my artist friends use this method.

We used to use white gouache for things like this. Another good option worth trying is using http://www.goldenpaints.com/products/gesso-ground/grounds-for-painting---drawing. It'll let you cover your markings--it's quite opaque--and still take wet media on top once dry.

Eh, I think trying to paint with white or ground over your blue lines will just look awkward. Use a light box. Non-photo blue isn't intended to be used this way; it's for black and white drawings that will be reproduced.

There's also http://www.dickblick.com/products/saral-wax-free-transfer-paper/ or the age-old trick of going over the back of your linework/photocopy with a soft graphite pencil, and then embossing the graphite onto your watercolor paper by retracing your linework with a ballpoint.