I don't know about thining it out, I mena, that would make it more runny. and runny paint usually makes blurry lines or goes under the stencil. I would use a thicker consistancy. Like, the consistancy of acrylic paint in the fancy expensive tube. I would probably do the stencil with an up and down motion with a foam spong thing, and do it really slowly. Other than that, I don't know. I've only done a super large stencil once (about 5 feet each side) and I could only get crisp lines with spraypaint, but then the paint wasn't strong enough.