V. Filleting and Taping
Building the Core Sound 20
March 6, 2008
NOTE: This article is divided into chapters. Click here for the Table of Contents.
The method of construction we're using for our boat is called "stitch and tape." I mentioned the stitching (wire ties used to temporarily hold the structure together) in one of the previous chapters. The taping part of the equation is actually a two-step process of laying an epoxy fillet (a contoured bed of thickened epoxy spread along the seams) and covering it with several layers of epoxy-saturated fiberglass tape (for strength).
Epoxy fillets can be fun to make, but they are also tedious and time-consuming. We cut a rounded edge in the end of a plastic putty knife to use for making nice, smoothly-contoured fillets, but our first batch came out a little too small. No matter. The next day, we roughed them up with 80-grit sandpaper and built the fillet up with more silica-thickened epoxy. Since the smaller fillet was enough to hold the panels in shape, I also removed the wire stitches before sanding and building up the fillet with more thickened epoxy. That way, the wire holes would also be filled in the same step. In retrospect, this worked so well and produced such smooth fillets that I might do my fillets in two steps deliberately the next time around.
Neatness definitely counts while you make the fillets. If you leave a mess of epoxy dabs and drips, there will be a lot of sanding to do later. So, after each fillet, we ran along the edges carefully with a plastic scraper to scrape up any spills or excess.
One mistake about epoxy fillets we learned the hard way: If you "overwork" the epoxy fillets too much (spreading and smoothing them over and over again, like an incurable perfectionist) you run the risk of causing them to sag. This happened to us, and at first we thought it was a problem with the amount of fumed silica we were mixing in to thicken the epoxy, but when it happened again (after adding a lot of fumed silica to produce a very thick epoxy), we realized it had something to do with scraping off and reseating the thickened epoxy one too many times, at which point it becomes strangely plasticized and prone to sagging. In our case, we just scraped off and threw away the sagging epoxy before it cured. I hate to waste epoxy, but there's no sense causing sanding hassles later. Better to start fresh with another batch.
Before the fillets were fully cured, we laid down the fiberglass tape. The plans call for three layers of 10 oz. fiberglass tape, but we used six layers of 6 oz. tape instead. That means our seams will have an extra 6 oz. of fiberglass on them, and we overlapped each layer to ensure a very strong joint. We used 6 oz. (instead of 10 oz.) tape because Raka quoted us an excellent per-roll price on the 6 oz. (and didn't have any 10 oz. in stock). However, I have also heard that more layers of lighter-weight cloth will yield stronger, stiffer results than fewer layers of heavier-weight cloth. So, as an added benefit, our seams may come out stronger, too. The only drawback to working with so many layers is that you have to be extra careful not to introduce bubbles of air under the tape as the layers shift around during the saturation process. Ours came out just fine because we took our time and saturated only two layers at a time, which is much easier than trying to wet out all six layers at once.
As with the fillets, neatness counts while laying the fiberglass tape. If you leave drips and runs of epoxy on the surrounding plywood, you will have a lot of sanding to do later. So once again, when we finished taping, we spent an extra 20 minutes carefully tracing along the edges of each seam with a plastic scraper to flatten and smooth out any spills or excess. Believe me, this is time well spent.
For all the tedium and long-hours involved at this stage, filleting and taping is actually quite exciting when you step back and begin to see the boat taking shape. Gone are the unsightly wire stitches that love to snag your clothes or poke your skin while you're working. Instead, the smooth, rigid lines of a structurally sound watercraft come into view. It's a very satisfying feeling.
© 2008, Wesley Kisting