Cover the wing with tape when sanding the fuselage near the wing sheeting. I usually sand the nose doublers to shape before they're glued on. You'll still have to hit the edges with 320 grit to finish them off.

A couple things I picked up from Windy's videos may help. On blending a soft balsa top block into the plywood doubler at the nose, it helps to soak up the balsa with thin CyA. That way you're sanding more equally hard materials. At the wing to fuse. joint, a block that has a steep angle and comes to a point allows you to work into the joint without touching the parts you don't want to sand. I like making a block up with an angled end as well as a side for sanding places like that.

I just pay attention to what I'm doing. Where the sanding material is and, most important, the direction I'm sanding. No pressure or no sandpaper where I don't want it. Choice of paper helps.

I've never used metal files on wood, but I have a small rasp. I have used a small wood plane.

Covering areas with tape is a preventative for keeping sand paper off of an area. Especially rib tops. I've even used typing paper to keep sandpaper off surfaces.

I've masked with copper tapes for the nose ring and posted photos of that. With any masking, a couple of layers for starters.

I started a Thread, "Hand made modeling tools."

I was sanding a leading edge at the time and posted a few photos of simple tools you could make for that task.

I thought for sure there would be participation in that Thread, with all that building talent out there?
 
Here's the link, never to early or to late to share modeling ideas and secrets.

http://stunthanger.com/smf/index.php/topic,37009.0.html

This nose has many different woods, both hard and soft, plus wood grain in many directions, not to mention CA and epoxy.

The spinner was the key, but before the spinner was used as a guide, the nose ring edge was masked with copper tape.

To blend materials of varying hardness, use as coarse a paper as possible, with as little pressure as possible.

    Brett

I think it's all good advice.  I tend to think in terms of using sanding blocks and paying really good attention to what they're touching while I'm stroking.

I've found that to work well especially if glued to a sanding block.

  Oh, absolutely, you should only use loose paper in very rare conditions!

     Eric is also right about Perma-Grit tools. Those are absolutely invaluable.

     Brett