Abrasives have many uses aboard, from removing paint and varnish to polishing out scratches in gelcoat. Sandpaper is an almost universally understood term; but sand, in fact, hasn’t been used as an abrasive for many years. Picking the correct sandpaper is a little more complicated than simply popping
into the local hardware store. Abrasive papers come in a plethora of types, grits,
and sizes to suit both hand-sanding and power-tool applications, so it pays to
know what to ask for. Buy the wrong stuff and you could be frustrated with the
results. To help you make wise choices, we’ve put together this handy guide.
Look on the pack of any sheet of abrasive paper and you’ll notice a number – 120,
for example, or 180, or 220. This number refers to the grit size, the actual physical
dimensions of each particle glued to the backing. The larger the number, the smaller
the grit: An 80-grit paper would be used for the swift removal of material, whereas
a 400-grit abrasive would be used for fine finishing. Grit sizes are universal, so a
120-grit from one manufacturer will be virtually identical to 120-grit paper from
another. The coarser grits, from 24 up to about 80, are used for stock removal and
will cut away at the surface very quickly. Grits from around 100 to 240 are best for
general smoothing, while grits from 300 and above tend to fall more into the polishing
There are several different and distinct types of abrasives (the actual material that’s
stuck onto the backing paper). Aluminum oxide is the most abundant, comes in a
variety of different colors depending on the manufacturer, is available in a wide variety
of grit sizes, and can be purchased in rolls or sheets for hand sanding or may be precut
to suit many popular styles of sanding machines.
Silicon-carbide abrasives tend to be black or gray in color, are waterproof, and can
be used wet or dry. Silicon carbide, very hard and long lasting, is ideal for the wet
sanding of varnish or of paint between coats. When used wet, a slurry is formed that
further speeds up the cutting process. Silicon-carbide abrasives can be purchased in
very fine grades, ideal for sanding gelcoat before compounding and buffing.
Harder, and therefore longer lasting than aluminum oxide or silicon carbide, zirconia or ceramic abrasives have higher price tags and are unlikely to be available except
through specialty retailers.
Open Coat Or Closed Coat?
Open coat and closed coat refer to how
closely the grit is spaced and has nothing
to do with the actual grit size. An open-coat paper is probably best for sanding
paint and varnish surfaces – with a bigger
gap between each of the abrasive crystals,
it’s less likely to clog up. On the other
hand, because there’s a greater number
of grit crystals per square inch on closed-coat paper, it will most likely cut faster on
harder materials, such as metals, as there
are more pieces of grit to do the actual
work of abrading the surface.
Do any boat DIY project, and you’re almost sure to use some
sandpaper. Which sort you pick will depend on the job at hand
Sand & Deliver
BY MARK CORKE & ROGER MARSHALL
84 | Do It Yourself