Do you paint? Look here, need help

I'm doing a painting for Heathen's school for Christmas. It's a winter window scene, looking outside to the snow, acrylic on canvas.
I want to do a grey wash/weathering to it.
What's the best way to go about doing that?
It needs to be done Monday night.
Thanks

Oil, acrylics, or watercolor?

for weathering Id use a crackle medium

Haha...I just saw acrylics on canvas.

This has some good tips:

http://www.ehow.com/info_12101195_techniques-weathering-vintage-look-canvas-acrylic-paint.html

Quoting khigh:" Haha...I just saw acrylics on canvas. This has some good tips: http://www.ehow.com/info_12101195_techniques-weathering-vintage-look-canvas-acrylic-paint.html"

Saw that one and liked it. But I figured I'd ask on here.
Thanks