I never have used Thompson's wood product other than decks so I have no experience there but is something I would try. I am a woodworker by trade and my recommendation is to stay with a sanding sealer matched with the same exterior varnish or a varnish mix only. I have had good luck using a reduced varnish, about 1/3 of what they say to clean your brush with, mostly mineral spirits and apply 2-3 coats until the pores are filled. Then sand with about 220. If you need to apply more do so until when sanding you get a no pocks completely smooth white dust surface. At that time go to full strength and apply as many coats as desired for finish still sanding between coats. You can reach the desired luster you want using this process. Good luck and have fun
Dan
