Sanding basics

Model Airplane News, Nov 1998 by Alfafara, Mariano

Secrets to surface preparation

IT HAS BEEN said that you can measure a builder's skill level-without seeing a model-by examining the sanding tools in his workshop. I have visited many workshops, and I agree that the best builders/finishers use only premium-quality sandpaper and have made a wide assortment of specially designed sanding tools. Of all the skills that we modelers must master, sanding is probably the most important.

Before we can develop good sanding techniques, however, we must be able to select the correct materials for the job. How do you choose the correct sandpaper for a given task? Novice modelers may find the many choices somewhat confusing: production paper, garnet paper, or flint paper? Silicon carbide, emery, or aluminum oxide? Open coat, closed coat, or no-load? What size grit or grade? Wet or dry? What is A, C and D paper?

A little baffling? Possibly, but only if you can't identify the various sandpaper types and designs. Therefore, our objective is to simplify this selection process by learning about abrasives and their designated applications. The next time you're at the hobby shop and need sandpaper, you'll be able to say something like, "Give me a couple of sheets of open-coat, aluminum oxide, 320-grit, on A paper."

The sandpaper that we model builders use is technically called "coated abrasive sheets." To simplify this discussion, we will continue to use the common misnomer: sandpaper. Three basic components make up sandpaper: an abrasive, a bonding agent and a backing (see Figure 1).

ABRASIVE MATERIALS

Natural minerals (garnet, crocus and emery). Garnet is a widely used mineral of medium hardness; it has a sharp form and good cutting edges. It's a good choice to use with balsa and other soft woods. It is paper-mounted, orangebrown, economical and readily available. Crocus and emery are cloth-backed abrasives that are designed for metal finishing. Other than polishing the occasional small metal part, there is limited use for these products in our workshops.

Synthetic minerals. Aluminum oxide and silicon carbide are generally the only two synthetic minerals that suit our modeling applications.

Aluminum oxide is gray-brown, or sometimes gold. It is extremely hard and resistant to wear. 3M markets this abrasive under the trade name of ThreeM-ite (not to be confused with their TriM-ite). Norton markets its aluminum oxide under the trade name Adalox. I've used both of these paper-backed products on hardwoods, resins and paints with equally exceptional results. Production paper is made with aluminum oxide.

Silicon carbide is the hardest and sharpest of the minerals commonly used in coated abrasives. It is brittle and fractures into sliver-like wedges as it works. You could say that it sharpens itself as you use it-pretty cool, huh? It's ideal for both ferrous and non-ferrous metals, e.g., steel, aluminum and brass, and also for plastic, wood, resin, rubber and both hard and soft paints. Silicon carbide is superior to other abrasives in its capability to penetrate and cut faster under light pressure. This characteristic alone makes it very well-suited to modeling. The 3M trade name for silicon carbide is Tri-Mite; the Norton trade name is Durite. Both brands are gray. Although it is more expensive, silicon carbide is certainly the abrasive of choice in my shop.

An additional note on types of abrasives: remember flint paper, that sandpaper we used in shop class at school? Well, if you have any of that stuff in your shop, throw it away or give it to someone who does craft projects.

COAT TYPES

Abrasives are coated onto a backing by two methods: open coat and closed coat. Open coat indicates that the abrasive grains cover 40 to 70 percent of the backing. If the material being removed has a tendency to load or clog, an open-coat sandpaper is the proper choice. Loading is the tendency of the material being removed to build up on the abrasive grain. We should use open-coat sandpaper for most of our modeling needs. Closed coat indicates that the abrasive grains cover 100 percent of the backing. A closed coat is the proper choice when loading is not a problem, such as in metal finishing.

WET SANDING

For wet sanding, a waterproof bonding agent is used with silicon carbide. The 3M trade name is Wet-or-Dry Tri-M-ite; Norton calls it Tufbak Durite. Both are black. As their names imply, they can be used in either wet or dry sanding applications. These products really excel at wet sanding, however.

With the non-clogging action provided by water and the superior cutting ability of silicon carbide, these papers can quickly remove a lot of material.

The makers of Hobby Poxy* paint recommend that you do not dry-sand their paints with wet/dry paper. They feel that the waterproofing agents in the paper could get on your finish and interfere with paint adhesion. I have used this sandpaper both wet and dry and have not had any problems, but caution is advised. In any case, you should always wipe the model surface with alcohol or thinner after sanding to remove any residue.