BAR with balsa fin/cone finishing question? As a kid, I didn't worry much with filling the balsa grain in fins and nose cones and if I did, not much. Build it, paint it and launch it! Next!

As a model builder again, I would like to finish my models to a higher standard and have gained good info from this forum. I am currently using the Elmer Fill/Finish product and have had good results, not great? It seems that no matter how many coats I apply, some grain remains - especially on a nose cone I'm currently working on. Does the balsa need to be sealed before the filling process starts? Seems like the balsa is soaking it up like a sponge? Thanks in advance for any advice/info!