I sanded the foundation just enough to remove the roughness and excess spackling from the base.

After wiping it down with a damp cloth, I patted on some matte gel medium. This added texture that was more in scale than the original spackling roughness.

I then painted a base coat of neutral grey. This sealed the finish and gave me a good base for turning the mdf into concrete (see previous post on my technique).

After some washes, the finish is now varied and has more depth.  I sprayed the base with Testors Frosted Glass spray to move the acrylic paint sheen. I'll add dust and dirt during the landscaping phase.

Now it will sit until I put the building on it.