I checked out the link, and it's just vinyl spackling compound, This is what Allen Brickhaus would use under thinned finish cure epoxy for his rattle can finishing system. This it with water into a slurry solution, brush it on, then sand off when dry.Not sure how dope would go over that but worth a test on some sheet balsa. Then you could flex it a bit when finished to see how flexible it is. At this point,add some talcum powder or corn starch to some dope for a filler, sealer coat. That is what they did in the old days.
This makes me think of something I have never seen asked about and that is Parks pigmented sealer/primer shellac. I think it can be thinned with alcohol. Used as a filler/sealer before staining wood work and such. What the heck, people have used lots of other stuff!
Type at you later,
Dan McEntee