I built cradles like that on my strong back to support the hull of my stripper while I built the deck. I had the hull glassed but not varnished. The boat sat on the cradles over this past summer, and I ended up with darkened wood where the sunlight coming through the windows and garage door hit the hull. Now I have some lighter colored stripes where the boat rested on the cradles, and lighter patches where the cradles blocked the light. If I did it again, I would be sure to cover up the boat with something opaque when not working on it.
Beware of this problem if your boat will sit on the cradles for a while.
Ken F.