1999
More True Grit
by Phillip Jacobs
'True Grit - Everything You Wanted To Know About Sandpaper for Boatbuilding' was the topic of the MCA General Membership meeting, held at Midwest Mountaineering, Thursday October 15th.
Galen Fitzel, Eugene Royster, and Marv Schifsky, technical representatives from 3M, started with the 'ABC's' of sandpaper, and how these components are used in various combinations for different sanding tasks. 'Abrasives' are the mineral or synthetic grit that does the cutting. 'Adhesives' or resins are used to 'bond' the grit together, or to the paper, cloth, or film 'backings'. 'Coatings' may be applied over the abrasive for special performance characteristics.
The good news is that the basic aluminum oxide sandpaper available at most hardware stores and home centers is pretty good, all-around stuff for sanding plywood forms, cedar strips, cured resins, and finishes. Certain abrasives and adhesives are better for machine sanding, and some coatings help prevent clogging with certain finishes, but these products may be slightly harder for the casual user to find.
One point of interest discussed was the grading system used for abrasives. '80' grit, for example, means that the abrasive minerals pass through a sieve with 80 wires per inch in each direction; this is why finer sandpaper is graded with a higher number.
Other codes and terms printed on the backsides of sandpaper were also deciphered. 'Open coat', for example, means that the abrasive grit only covers between 40 and 70 percent of the backing, allowing for more aggressive removal of both the stock and the dust created, much like saw blades with fewer teeth and larger gullets are used for rough cutting. 'Closed coat', means that the paper has 100 percent abrasive coverage for smoother sanding.
A number of innovative, abrasive products were demonstrated, including one which will remove paint from a board without abrading the wood underneath, and a pad which removed pin stripes from autobody material, without scratching the finish below. Participants also had an opportunity to pose technical questions to the 3M reps, and left with some samples of products to try.
Time constraints prevented us from exploring all of the products and tools available for sanding those hard-to-reach contours inside the hull. Nor did we examine and discuss the home made sanding tools participants brought, and hull sanding techniques developed through years of experience. This, clearly, needs to be the topic of a future meeting. Our thanks to the folks at 3M and MWM for co-sponsoring this one.
