Chris, I feel your pain! Getting a smooth surface prior to finishing a balsa model is very time consuming but it sure pays dividends in the end. The difference between a so-so finish and a good one comes down to sand paper! Of course, being careful during the assembly process really helps so that a great deal of sanding/filler is not necessary. I use a light weight spackle or light weight model filler on the bare balsa. Whatever you use, make sure it sands as easy or easier than the balsa, otherwise you are just begging for trouble. Whenever possible it is best to sand with a sanding block when trying to even out the surface. When dealing with raw balsa, hand sanding generally leads to more lumps and bumps than it eliminates! And holding the surface out in strong light is a really great way to find imperfections. But the biggest problem I see on many flying models is that guys just stop sanding too soon. I guess it comes down to what is “good enough” for each individual. I probably have not told you anything you didn’t already know but there is my .02 cents. If you have any other specific finishing questions please post them and I’d be happy to share what I know. (which probably isn’t all that much!) Best regards, Chad Veich.
PS- Before anyone else points it out, I know EB-B should not have invasion stripes or full span wings! However, the customer is always right.