I put a few filler coats of nitrate/cornstarch to fill the grain then cover with 1/2 oz glass cloth and SuperPoxy top coat. Those were the days of 70% nitro.

I think the take on modern silkspan is the weave is too open and it takes allot of weight to fill it. Silkspan or polyspan is used to add strength and to cover open bays. If your wing is stiff enough I would just fill and finish it without using anything.

MM