CL Aerobatics
BOB HUNT, Model Aviation's aeromodeling editor, recently wrote that in our magazine environment we revisit topics every three years or so. I've noticed a tendency to think in that cycle unconsciously, so this time I'm going to revisit one of the most important skills in building models: sanding.
I've touched on some of the points I'll cover in various columns, but I'm going to try to pull it all together this month.
Basically, I'll discuss two areas: materials and techniques. You may not agree with everything, but I hope you'll give some of the ideas a try.
My dad, who always supported me but never built a model, had a saying: "Never send a boy on a man's errand." I guess he got that from traveling throughout the world after World War I, to such places as Australia, Tasmania, and New Zealand.
That saying also applies to using sanding implements. We have various grits of paper, film, and tools designed to do assorted jobs. Use the grit/material that will cut the subject surface efficiently and effectively.
A poor application is performing initial part sanding with 400- to 600-grit paper and a lot of pressure. The opposite would be to sand paint with 80-grit paper. Go to another grit if your selection does not cut reasonably quickly. The same applies to the type of paper versus the material.
Materials
The material you are working with is the determining factor when selecting the type of paper and grit. By far the best for working contest balsa is garnet paper, used dry.
The big hardware chains carry garnet paper in varying grades in 180–280 grit, which works well for most of our applications.
Garnet paper is designed specifically for raw wood. The abrasive cuts cleanly in all grades with minimum pressure (a point I'll get to in more detail later). The nice feature is that even though it cuts well, there are usually few scratches you have to fill.
This paper is best used with sanding blocks—preferably glued to the pieces. The paper is durable and has a long life if its use is restricted to raw wood.
Dry versus Wet
There is no choice until you have paint on the airplane—you must go dry. Then you have some decisions I'd like to put a different spin on. Consider the backing on the paper as a factor in selection.
Wet/dry paper backing is stiff in the dry state—especially in the coarser grades. The grit will cut balsa and base coats well when dry; however, because of the backing it can very easily dig into soft contest-grade wood.
A better choice for the next finer grade of paper, after garnet paper has been used for primary shaping, is the 3M™ Fre-Cut™. On my cutter (400 grit), I was concerned that it would cut too much and that the subsequent coats would not cover the scratches. Was I wrong! The Fre-Cut™ does a great job—especially with Brodak Clear, which powders after drying several days.
The manuals for the use of this type of paint specify using 400 grit to sand the first coat after 72 hours. The Brodak Clear flows well over the sanded surface. Even though 400 grit is preferred for smoothness, I've used as coarse as 280 to sand imperfections.
This is extreme, but the Clear covers well and the next sanding phase will permit buffing and polishing.
After the clear coats, it's on to the final finish-sanding. There is no question; you want to use wet/dry paper. There are a couple of ways to go on what liquid to use—water or Slickers (a brand of quality automotive paint M-600).
For water, add a few drops of good dishwashing liquid, such as Ivory Snow®. Select a quality dishwashing liquid, because its primary purpose is to break the surface tension and get the paper to cut. A cheap liquid will probably not have the ingredient to break the surface tension.
Slickers M-600 is a degreaser; it will allow fine paper to cut very well. I really like the product, but be cautious; it is a strong chemical. Good ventilation and masks are required. During use you must keep the surface really wet or the sanded material will clog the paper.
For the final sanding, I use two automotive paint stores and get a variety of the 3M™ Imperial series wet/dry papers. Newer finer grits are being released, but 1200, 1500, and 2000 are good choices. Get a few of each.
Your choice of initial grit should be the finest that will cut reasonably and finish with 2000 before polishing. If you’ll be able to tell how the paper is cutting. Don’t force it.
Believe it or not, there is skill involved in sanding models. That there is a difference is obvious when you look at a variety of airplanes varying in finish.
Check out both the lines of the surface. How does the spinner match the contour of the fuselage? Does the wing shading ripple where the glue joints occur?
Using good technique when you sand will save time and trouble.
Tools
Before you start, make or acquire some tools. Please try the carbide-coated steel tools. They come in flat, semi-round, and round, in coarse, medium, and fine grits. The medium works well for shaping and the fine works well for smoother work. Fine also works exceptionally well for working epoxy/carbon parts, such as propellers.
You will also need several extruded-aluminum sanding blocks. At the start of each major section, I set up a series of graduated grits and lengths. Glue the paper to the aluminum with 3M™ 77 spray adhesive. The blocks can be cleaned and restored with thinner.
The choice block for sheeted surfaces is Styrofoam®. Approximate size of the sanding surface is four inches by ten inches. The block should be thick enough to be stiff (at least two to three inches).
The 3M™ 77 spray adhesive works fine for attaching the sandpaper to Styrofoam®, but it is a one-shot deal; thinner will dissolve the foam’s surface if you apply too much.
There is also need for unusual shapes. Acquire the X-Acto™ brand sanding shapes. The boxed set is approximately $6; it's money well spent.
Techniques
Achieving a lightweight structure with a treated sheeted surface area is very tough to get right. It’s common to see many dihedral
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Transcribed from original scans by AI. Minor OCR errors may remain.
