A bit about primers. I like Klass Kote high build epoxy primer. Not only does it sand like silk, being epoxy, it starts to build a bit of a hard (harder) surface, There is a lot of misinformation about primers. As I said, the biggest issue you have to deal with in a silkspan job is filling the balsa grain and pours under the silkspan covering. What some guys will do is shoot on a coat of primer, sand it off and shoot on another with the belief if they just keep laying on primer it will fill the grain. That is true to a point, but I can assure you with this method there will still be some extremely small (and sometimes large) grain that will not get filled.

This is why. Take a bowl of water and shoot your airgun straight into the bowl. What will happen is the water will blow back into your face and the bowl will be empty of water. The same thing happens when we try to fill grain by laying on more primer. When we shoot the primer on the surface, the air from our gun blows the primer back out of the hole and builds up around the edges like a volcano. So when we sand down the primer around the edges, the hole remains. We can lay on massive amounts of primer and some of the grain (holes) will never be filled.

I like to let the Klass Kote primer cure for 24 hours. I wet sand it with 400 back to where I start to see the grain of the wood underneath. It's important to not go too far and cut through (which we all do a little). At this point I want the primer cut back a long way, but I still want a fairly opaque, white surface. The reason I don't cut it too far, is it's easier to find unfilled grain. Once any remaining grain has been filled I can cut it back further.

Here is the Klass Kote primer:

http://www.klasskote.com/product/KKA100