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You are sanding. It's been a half-hour already but there's still four yards of hull to go. The garage is filled with a choking, gritty mist of epoxy particles, and your nose is inflamed. Your father's old 1/3rd-sheet sander, last used in 1979, is emitting an exhausting, excruciating whine that combines the aural qualities of fingernails on a chalkboard and a loose fanbelt. The sandpaper seems to fill as soon as you change it, but feeding paper to the machine as soon as it stops cutting is tiresome and expensive, so you just press down harder and use the corner of the pad to stab at those rock-hard globs of epoxy. Hours later, you are proud of your thoroughness; you can run your hand from bow to stern without finding any major lumps. Certain that you will never touch a sander again, you stir the $25 can of marine paint and swipe it on. You stand back and are dismayed to see that the deep gloss of the paint has revealed a patchwork of epoxy runs, sags, and bumps. You've spent two hours sanding, for this?
Sound familiar? Here are some ways to avoid sanding misery and ensure a perfectly smooth hull. These remarks refer to sanding epoxy-coated wood, a special case demanding different treatment, as you will see.
Don't Breathe The Stuff. Epoxy dust is toxic, period. At best, expect nasal inflammation, at worst, throat cold symptoms, or sensitization that could keep you out of a boatshop for good. Good quality disposable dust masks such as 3M #8710 will get you through one boat, but consider a real respirator with cartridges. It feels comfortable on your face for long periods and allows normal breathing, whereas the dustmasks always feel hot and scratchy.
Shut Out That Noise. Ear plugs, or better, industrial ear protectors that look like headphones, are a great luxury and will increase your endurance tenfold.
Invest In A Nice Sander. I don't have the greatest tool collection, but I do have a complete range of high-quality sanding equipment. You don't need anything fancy or even more than one sander to build a kayak. If I had to choose one, it would be one of the "random orbital" sanders with a 5-inch pad. Random orbital sanders are a relatively new type that can move a lot of hardened epoxy for their size and cost. My favorite random orbitals are the DeWalt DW421 and the Bosch B7250.
Keep The Sandpaper Sharp! It upsets me to see someone trying to sand a hull with a single piece of sandpaper. Continuing with dull paper makes it impossible to cut a uniform amount of material off the boat and what you do accomplish requires three times the effort. Worse, sanding with dull paper builds up a lot of heat between the paper and the hull, softening the epoxy and filling the paper even more. Budget for sandpaper from the beginning. On the first pass at a freshly epoxied 17-foot kayak I'll change the 80 grit paper every three feet! Yes, sandpaper is expensive, so don't buy it at the home center. Instead, buy Sanding Discs from us in bulk.
The Pad Must Be Flat! Here's the one tip that will make all the difference. Do not, I say, do not sand on your hull with the edge of your sanding pad. We've all done it because you can remove material more quickly that way. This is because on a disk grinder the outer perimeter of the circular pad is spinning so much faster than near the center, and because it seems natural to focus on a pesky glob of hardened epoxy with the fast-cutting edge of the sander. Unfortunately, the unforgiving sheen of marine paint or varnish will reveal a hull covered in crescent-shaped indentations where you cut too deeply. Yes, it will take longer to mow down those bumps with the pad kept perfectly flat against the hull, but the reward will be ample. Use a sanding block for the chines and keel; the hard corners will smooth off quickly by hand.
Just How Much Sanding Are You Suggesting Here? If you have sanded the whole hull with the sanding pad kept flat, you are finished when the epoxy surface is a uniform cloudy color, with no little dark or glossy indentations. I use 80 grit at first, then 120, but if you are going to skip a primer coat of paint, you should go over the hull with a minimum of 220 grit to eliminate the tiny swirls cut into the epoxy by the rough abrasive, which wil be visible beneath marine paint. Wet sanding? Save that for between coats of finish paint, using a sanding block.
Unfortunately, none of these tips will really make sanding go more quickly. However, you have to live with your paint or varnish job for a lot longer than with the tedium of sanding, so the payoff is high. I don't know who originated the remark, "boatbuilding is 95 percent sanding," but he was right on.