Opaque projectors for transferring small sketches to large canvases

How do you take a tiny sketch and make a huge painting, without losing the liveliness of those first marks?

For me, the answer is:

Use an opaque projector. 

Here, I have painted a background already, and I am tracing my drawing using white paint mixed with acrylic glazing fluid.

Why use white paint? 

Mistakes can be simply erased with a clean, wet brush--paint wipes easily off a less-absorbent surface like this. The white paint is a better choice than black, since a white under-layer lets translucent colors shine more brightly when they are layered over it.