is pretty clear the lower the grit number the more material is removed per each stroke. Also the more material that is remove the less precision you will have. If one puts a piece of sandpaper under a microscope they would see the following;
Also if one puts the piece that is being sanding under the microscope they will also see similar mountains and valleys.
If one want to shape something precise then these mountains and valleys needs to be minimized. In engineering terms this is referred to as surface finish. If someone wants to make a wing .235" thick at the high point and wants the thickness to be precise to .001" then it should be clear one needs to be sanding with at least a 320 grit sandpaper (see table above).
Going Through The Grits
Woodworkers have used the term "going through the grits" to describe how one should properly sand. What this means is that you should start with low number grit sandpaper and as you get closer to the desired shape or thickness you start going up in grit number. Here’s my general rule of thumb;
|Grit||Distance From Desired Shape|
At the low grit numbers I find that the stones tear the wood and one needs to be careful not to tear the balsa wood which creates large valleys.
Making A Sanding Block
Making a sanding block is easy once you know how. Get a piece of balsa wood and cut it into a block. Shape the top and sides of the block to make it feel good in your hand. Next make the flat sanding portion of the block by using your flat sanding surface. Typically you want a very flat, smooth surface on the sanding block which you attach the sandpaper to. To get your sanding block flat and smooth, I use sandpaper, a aluminum plate and some double sided tape. Attach a large 8.5"X11" piece of sandpaper to the aluminum plate using the double sided tape and then just take your sanding block and sand the surface flat. See Photo 5. Be careful not to rock the sanding block as you move it across the sandpaper.
Flat Aluminum Surface with Sandpaper attached to make sanding blocks.
A roll of double sided tape is shown also.
Next take some double side tape or carpet tape and attach it to the flat sanded surface of the balsa block. Finally take the desired sandpaper (grit number) and attach it to the tape. Your sanding block is complete.
The sanding mechanics, the motion and pressure used to sand is something that is very hard to describe and really takes practice to master. I don’t worry about going across the grain, so I sand with circular motions. The most important thing is not too hard of pressure. Pressing down hard on the sanding block is easy to do since the harder you press the more material you can remove but pressing hard also cause heat and heat causes the wood to expand. Wood that is locally being heated and material being removed at the same time causes huge waves and warps. Related to this is how "used" the sandpaper is. Sandpaper wears out or dulls and dulled sandpaper make you press harder to remove material. Remember pressing hard is not a good thing so replacing the sandpaper on your sanding block frequently is very important. What I do is replace the paper on all my blocks when I start a new glider.
10 Basic Steps To Making A Glider Wing
- Choose a glider design, get plans and make templates and fixtures.
- Choose the piece of wood. Evaluate for:
- Trace out platform outline of wing
- Cut platform outline of wing
- Glue on Hardwood Leading Edge (optional)
- Sand the bottom of the wing flat
- Sand root to tip taper. See Photo 6.
- Sand highpoint to trailing edge shape. See Photo 7.
- Sand LE to high point shape
- Sand Upsweep