I am working on a document in Illustrator and I have some sketches that I'm using that are just strokes, there isn't fill. That was fine until I changed the background to grey. I want the sketches to have a white fill. I thought the pathfinder divide tool would work for this situation but it didn't, Unless I'm doing something wrong. Here is the image.
What I have done in the past is to group all the paint stroke art work together in one layer and then duplicate that layer. I would lock the top layer and then go and use "live paint" on the bottom layer. The live paint tool will remove all the paint strokes. This is why you need to duplicate the layer so you can. Have the stroke appear over the fills. Here is a. Tutorial on the tool.
This explains how it works. It is really simple once you get the hang of it. Just be aware you might need to create a couple patch layers for anywhere the paths are open.
The other option is it just to group all your artwork together and make a new layer below. On that new layer you would have to pen out a simple shape that would be the fill for your artwork. I hope this makes sense.
Good luck and I love you illustrations.. Nice work!