Tools - The Essentials, Sanding Blocks Part 3

After some sanding you should check for build-up on the sandpaper.
These little nubs of filler/primer can get large enough to scar your balsa when sanding!

On the right is a soft brass wire brush I picked up at Home Depot. Be sure to look for a soft brass brush like you could use on suede shoes.

Budget TIP: In the sink, run some water over the sandpaper. Using circular motions, brush off the loaded up gunk.
Your sandpaper is as good as new! Well, close to new and reusable anyway. Each time you clean off the surface the sandpaper becomes a slightly finer grit. You can also wear down the brass bristles over time. My first brass brush lasted for years.

After removing most of the filler/primer from the tube or fins with 220 grit on the block, I lightly sand and smooth out any scratches with a small piece of 400 grit. (Shown below on the left.) This takes off most all of the gray filler primer filling any remaining balsa grain or tube seams.

Here's the same piece of filler/primer loaded 400 grit before -
and after brushing out the filler primer.

These new premium sandpapers are more expensive than the old black sandpapers. But, the new stuff can be brushed off and used over and over again. I'll probably use one single small piece like this to sand a Big Bertha sized model, brushing it off every time it gets loaded up.

OPINION: CWF does not seal the wood! Old style Sanding Sealer does seal the wood surface. Sanding Sealer or Balsa Fillercoat is hard to find today and expensive when you do. Body tubes are rarely sealed before spray painting. Why bring this up?
I sand my models dry - no wet sanding (if I do it at all) until the model has a fairly heavy, dried coat of enamel.
CWF is water based, the balsa can still absorb water when wet sanding and swell up. Kraft tubes will absorb water. Wet sanding will slow down the "loading" of sandpaper with filler/primer. If you decide wet sand, do it only after color coats are sprayed using very fine sandpapers. Keep the water away from the open ends of the body tube and engine mount.
Watch a video of someone clear coating a guitar or piece of furniture. The wet sanding is only done after a few color or clear coats have dried. Be very careful not to sand through the color or clear coats.