This looks like new territory. The standard sealer for Koverall has been nitrate dope, three coats or more, depending on the builder's wishes. I think the Lusterkote line includes a primer-I would use it if available. HOWEVER-a warning from several peoples experience (I have not used the product). There have been numerous reports of cans of Lusterkote that fail to spray properly and some reports of colors (red that I am sure of) that faded badly to a strange black color after exposure to the sun. Build some test frames and try your combination before committing it to the plane. Better to find out something isn't compatable before you have to clean up an airplane.

BTW, you likely wouldn't want to overdo the filler on a Cub because the fabric texture normally shows in the paint. Monokoted Cubs look strange because they are too slick.