So apparently Murray decided to test this out today, and didn't look up when he heard gulliver playing by himself for a while. When he finally looked up, Murray found this:
Gulliver had gotten into the artist's gesso. Which by the way is completely water soluble. Phew!
I walked in the door just in time to find Murray catering him upstairs to the bathroom.
-- Posted from my iPhone, with apologies for any typos.