SpringFling Garage....Day 1

Here's what Greenleaf gave us to play with:
I'm not going to divulge what look I'm aiming for as I assemble this garage. Suffice it to say that from the time I was very small, my mother told me I was contrary. It either had to be my way, or I'd refuse to play. Apparently contrarian and contrarism are real words. In the sixies they'd be called "free to be me" people.

Being contrary means you have to be very careful when you pick which parts of the garage go where. I want my garage exterior to appear to be weathered wood and because the "boards" will not fit tightly together, I needed to paint the walls black. On hand I had black tempera & black acrylic paints. Which to use? Can't get the plywood too wet in case it warps. So I decided to mix my two blacks.
Imagine my amazement when they reacted & gave me a thick oozy gel. I've googled to try & figure out why, but am still at a loss. My garage parts, however, are a lovely evenly opaque black....just perfect for what I want.

Hint: