Wallering is the result of part of the surface being harder than the other. This could be epoxy glue joints, where hard wood meets balsa, or even harder sections of balsa. It doesn't matter if you are using a block. This came really aparrent when I was block sanding my car for paint. As soon as I hit metal, sanding stopped. If I continued to block sand, it would still "waller" (if you can come up with a better term let me know). With a sharp 60 grit paper it cuts through harder areas such as wood and epoxy glue joints without creating low spots. It is much easier to come up with a final shape.

Added: this is greatly effected by the underlying structure, where the wood flexes and where it does not. Even if you think it's not.

It also cuts down on the work load. Here Is my write up on fiberglassing:

http://www.rc-tech.net/fiber/fiber.htm

I cut the finish with 40/60 grit. It makes fast work and it makes it real even. You just have to smooth it for paint which is not a problem. Cuts the sanding time down by 1/3. Still had an "Automotive" finish when done.

I learned this the hard way years ago. I bought a Royal C-310. When I was building it the plans had me glue several big balsa blocks together. Afterwards it said sand to shape. I spent 30 minuts looking for the missing part in the directions till I figured that they really did mean sand it. I cant tell you how many hours I spent sanding that buger with 100 grit. After that I started experimenting with rougher sand paper. I found everone trying to push 100-120 grit for shapping and everywhere I turned all I could see was surfaces that look like waves and people who had sanded for hours. When I plank the Ziroli airplanmes I wouldn't even think of using 100 grit for shaping. I have also put iron on covering over sport planes sanded 60 gritt. You know what? No one could tell. I think if your sanding 600 grit, your wasting your time.


If I built smaller park flyiers, I might feel differently. The scale of things would make a difference.