Friday, November 30, 2012

Shop Talk: Hand Sanding
By Randy Davis


If you want a sharp looking pinewood derby car, then proper sanding is critical. Choosing correct grits of paper, selecting the right sanding accessories, and applying the proper techniques are all necessary for getting a silky smooth block that is ready for painting. Let's take a look at each of these areas in turn.

SANDPAPER TYPES AND GRIT
For wood sanding "garnet" and "aluminum oxide" papers are the most commonly used. Garnet paper (brownish-red color) provides a slightly better finish, but it wears out quickly. Aluminum oxide paper (generally silver gray, but can be other colors) lasts much longer, but doesn't provide quite as nice of a finish. One technique is to use aluminum oxide paper for the coarser sanding, and then use garnet paper for the final sanding.

Another type of paper is "silicon carbide". This type of paper can be used wet or dry, and is generally used for sanding between coats of primer/paint.

Sandpaper coarseness is rated by "grit", with lower numbers indicating coarser paper. Generally, a coarse paper is used for initial shaping and removing saw marks. Then finer papers are used to remove the marks left by the coarser paper. A common grit progression for pinewood derby work is: 60, 100, 150, and 220.(1) A quicker progression is 60, 120 and 220. Generally, after 220 grit paper, car bodies can be primed. However, some people like to go a step further and use 400 grit paper before painting.

SANDING ACCESSORIES
Sanding is easiest and most effective when the sandpaper is held in a sandpaper holder or wrapped around a piece of wood. For sanding flat and outward curving surfaces, a padded sandpaper holder(2) is the way to go (see Figure 1). This holder is designed to securely hold one-quarter of a sheet of sandpaper. The padded surface improves the smoothness of the finish, especially with finer papers.


Figure 1 - Padded Sandpaper Holder

For initial shaping with coarser papers, if you need to ensure the flatness of a surface, then you can wrap sandpaper around a block of wood such as a pinewood derby block (see Figure 2). Use a standard stapler to fasten the paper to the block on one side - just don't sand on the side with the staples! Once you have achieved the desired flatness, switch to a padded sandpaper holder.


Figure 2 - Sandpaper Around Block

Another handy sanding tool is a large diameter dowel rod. By wrapping paper around the dowel rod, inward curves can be readily shaped and smoothed. If you don't have a dowel rod, then wrap some sandpaper around your index finger.

SANDING TECHNIQUES
First, before any discussion of technique, make sure to wear a particle mask and safety glasses when sanding. Filling your nose, lungs, and eyes with wood dust is no fun, and certainly not healthy. I have also found that sanding in the open air where there is a little breeze will greatly reduce the cloud of dust that tends to hover
around.

-- Stroke --
When sanding car bodies, where at all possible sand with the grain of the wood. This provides the best finish with the least amount of work. On the end of the block (such as the back of the car body), you can sand in any direction. But I have found that a circular motion works best.


Figure 3 - Sanding Direction

-- Progression--
Start with the coarser paper, making sure to achieve the desired shape and eliminate saw marks. Wipe or blow off the dust so that the surface can be inspected. If marks still exist, sand some more. Parents, you can help your children know how much to sand by marking flawed areas with a pencil. Then tell them to sand off the pencil marks.

When ready, switch to the next finer paper. Sand until all marks from the coarser paper are eliminated. Make sure to wipe off the dust and inspect carefully. Then continue the process until the last paper is completed.

One final step before painting is to "ease the edges". This is done by taking 220 grit sandpaper or a sanding sponge(3) and sanding all edges of the car to eliminate the sharpness. Easing the edges gives the car body a nicer feel, but mostly it improves the final finish as it allows the paint to bind better to the edges of the car body.

(1) Maximum Velocity offers a Sandpaper Assortment containing 100,
150, and 220 grit papers. Our pre-cut car bodies have already been
sanded with 60 grit paper.

(2) You can find a padded sandpaper holder Here.

(3) Sanding sponges can be found Here

From Pinewood Derby Times Volume 12, Issue 5

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