Last Saturday, I was out at our RC field and heard one of my friends tell about how he gets a smooth surface on balsa without much sanding.  He mixes light spackling (the kind that comes in a plastic jar and is almost weightless) with water to a milky consistency.  He then brushes it on the plane with a foam brush.  When dry, he sands with medium to fine sandpaper, and all the grain lines, etc. are gone.  He claims that he applies his film covering directly without using clear dope or balsarite, and the film adheres just fine, but I think I would feel safer using a base coat of some type.  Should work well for silkspan and dope, or fabric.  I might give it a try.

Glen Wearden

If you just mix the spachtle with water it gets even softer and tends to sand out of any dings or cracks, which then show through the covering.

I've had better results thinning the stuff with water-based polyu varnish(Min-Wax, Varathane).  1 tbsp per cup is usually about right.  If you add too much it will sink to the bottom of the container overnight.  You can mix it back in or add a more spachtle.

If you are trying for a 20 pt finish this is probably not the way to go.  But for a sport plane, or just plane in a hurry it works pretty well.

This sounds like the stuff Hobby Poxy used to sell until the government stepped in.  It was just like putty out of the can.  Flying partner would just put it on with a spatula as thin as he could get it.  Once the plane had an even color to it he would set it aside for a day and then start sanding.  Created more dust than dope and talc.  One coat of clear dope to seal it(brushed).  Then go with his colors.  After several years never seen any sign of wood grain.  This was done on solid wood surfaces.  He said it would not work on fabric. 

I have actually done this on the Cardinal pictured below - first tho, you have to spray the balsa to get it to swell a little (just lightly spray it) then put the spackle "paste" on it.

the finish comes out really nice...... I got the idea from Heman Lees aeromaniacs site .......