Painting on a burl

I was asked to paint a picture of a farm on a wood slab, which might be from a burl, and it's probably walnut. The slab was cut in a sawmill, maybe 20 years ago. It has changed hands, and discolored. When I sanded away the discoloration and saw the grain pattern, I was impressed. I knew that I had to incorporate the grain into the picture I was painting. I decided to try something I haven't done before, or recall hearing of others doing it. I wanted to make the picture opaque in the middle, but become a stain toward the edge, which would allow the grain to show. The grain and the picture would merge and blend.
The clear coat layers would have to be applied in stages, so that the wood could absorb the stained part at the edges of the picture, yet there would also be a solid base for the lettering and outlines. The stain I refer to is just the same oil based lettering paints that I used in the middle, but thinned down considerably to make a stain.
bare wood
picture and clear coat
a closer view
picture blends to grain