I am building the Arado 76 from MAN plans. It is a light .26 size plane, about 3 lbs when done (hopefully).
While most of the plane is covered with Ultracote, the top is not,it is bare balsa. The plan instructions say to "fill the balsa so that it is smooth like metal, then paint.

I tried Sig dope sanding filler and it doesn't really do the job.
Can anyone suggest a filler that a)fills the grain; and b)is sandable. Thanks

Since you've already used dope-based sanding sealer, I would recommend using clear dope and adding talcum powder until it's fairly thick, then thin it to brushing consistency. Sanding sealer isn't really designed to fill wood grain. It's what it says, a sealer applied after sanding to fill the very fine scratches that are left.

Actually, there's no reason you can't add the talc to the sanding sealer to make it fill the grain better. Just thin it down until it brushes out smoothly. Allow the fill coat to dry well, sand it almost all away, then apply another coat. The idea here is to fill the grain and not have any more filler on the wood than is necessary.

You can also add talcum powder to water based polueurathane and it seals and sands really well. If you use Johnson's baby powder it smells great when you sand it.

Just a thought now that it is too late -

When I am cutting ffhlg wings, I rough shape (down to 0.5mm of finished) with 80 grit then 240 grit. At that point I give the wing two good coats of weak dope, the first 50/50 dope/thinners, the second 25/75. Leave to dry thoroughly then cut all the fluff off with 240 grit. Give another coat of 25/75, leave to dry off thoroughly then polish up with 800 grit. Then use the sanding sealer.

The problem (and I used to have this as well) is that the raw wood soaks the dope out of the sanding selaer like gang busters which means that most of it comes off again at the first touch of the sandpaper.