The mayo idea seems like a good idea. You can make mayo using vinegar, oil and egg using a hand blender. If you are going to use it to etch then you can adjust the vinegar content upwards to help with your etching. The fact that it will be thick will make it easy to brush on or apply with a q-tip. Good idea! I would suggest a piece of clear scotch tape (not packing tape) to cut the stencil and a sharp blade.