Rushton's shop used shellac under the varnish. They would double up on the shellac and cut down on varnish on rush jobs. I have used shellac inside Rushton hulls as a base coat and to attempt to achieve the color that you often find in Rushton boats. When I use it (inside a hull) I do not bother to thin it. It tends to go on really well as is. It penetrates well and it dries quite quickly. A canoe I finished this way over 40 years ago still has the same varnish on it. I don't see any signs of separation, bubbling, clouding etc. It looks really great. As with all other W/C hulls, it gets a coat or two of varnish once in a while as standard maintenance. I have also used shellac on the outside of a hull. It will cloud a bit if you leave the boat is in the water everyday for a week or two. I always pull the boat from the water at the end of the day but 6 plus hours of daily paddling followed by sitting on it's side in the damp evenings eventually takes a toll. Once the trip is over it dries back out and I usually touch up the dings and give it a coat or two of shellac before I store it. Canoes finished like this are typically "working" canoes so I am not very particular about it's appearance. A bit of clouding, a few scratches, whatever, it's all good.