Stitch & Tape

I see a lot of things in my travels around my clients, some good ideas and some surprising gaps in what I think is “common knowledge”. Reminders that common knowledge is no more common than the fabled common sense which in my experience seems to be anything but. Common that is!

One of the things that I thought was pretty well known, but from what I see should perhaps be brought up and aired every year or so is how to make a stitch and tape seam and hide the edges.

Now there are a lot of boats around where the builders know how this can be done, and their boats look the better for it, but there are a few who have either struggled to actually sand the edges of the epoxy saturated tapes down to a feather edge and have made great dips in the plywood along that edge, or who have just painted over what looks like a strip of sacking and gone boating.

This is for those who would like have an easy way of hiding that edge:

Now you will note that I mention WEST System products and that’s what I use, the guys here at Adhesive Technologies in Auckland NZ have over the years been very very helpful to me and I support them in return, but there are many equivalent products out there and they will, I am sure, do the job just fine.

I use West System Epoxy resin, with their usual glue extender and 410 microlite. Sometimes I even go to Microballons if I need a thixotropic (completely non sagging) mix but usually put glue mix and Microlite in at a ratio of one glue to three microlite by volume to just beef the resulting cured filler up a little in compressive strength and abrasion resistance. It still sands pretty well and does the job nicely.

I should also put in a plug for Chuck's Cheap Chuckaway Chip Brushes, (say that one quickly five times over! See them in his on line “shop”) as you’ll be throwing away about a brush for each half hours work.

I have shown the procedure that I use when taping seams and edges in cartoon form.

1) Lace the edges of the ply together, use plastic cable ties, plastic jacketed or plain soft wire (copper or similar) or even polyproplene string. I have used monofilament fishing line but that requires some expertise in tying the knots. Tie at quite close intervals, perhaps 15 times the thickness of the ply so the edges remain in place when you are working on the boat.

Keep the lacing close to the edge, no further from the edge than the ply is thick.

2) An inside view, showing that although the seam is open on the outside, it is a close fit on the inside edge.

3) Make a nice radiused spatula, I use about a 30mm radius ( 1 1/4in) for a boat using 1/4 or 3/8 in ply, a little bigger for 1/2 inch or more . Lay a radiused fillet of glue mix (no lightweight filler, just glue well thickened) along the full length of the area to be taped, straight over the ties and all. Tidy up really well as you don’t want any leftover bumps in the road.

4) While the epoxy fillet is still wet, paint on a layer of unthickened resin and hardener in a strip out to the line where the tape (s) will come to. This is a wet on wet operation, and on a hot day you need to get the tape on pretty quickly before the epoxy in the fillet goes off so don’t try and do each step for all of the boats bottom before going back and doing the next one. Better to complete the filleting and taping over a couple of metres then commence the next section.

Note that the paint brush can be used to give the epoxy fillet a final smoothing while you lay on the resin that will wet out the tape.

5) Lay on your tape using the paintbrush to ease the tape onto the resin covered fillet and ply, press it in gently and smooth it with the brush using more resin where needed to get full wetout. If you are using multiple tapes lay them on wet on wet until you have built up your full laminate, and make sure while doing so that you lay the tapes up staggered so that the edges do not coincide and build up a thick edge that will be harder to hide.

Its important to maintain a finish as smooth as possible.

6) A day or two later when the epoxy is crisp enough to sand* effectively go through with a sanding block with fairly coarse sanding paper and knock all the ragged spikes and lumps off the edges.

7) With a slightly flexible squeegee, I use one cut out of the side of a large ice cream tub or similar, go through and lay a wide and shallow fillet (of low density filler per the formula above) out from the edge. One layer of 6oz tape requires a 60mm wide fillet, two layers needs about 100mm ( 4in) as does a single layer of 10 oz. that fillet should be layed on a little high, and you can fill the weave of the tape at the same time. Clean up well to save the sandpaper time.

8) Sand off and paint. Don’t sand off too hard or you will be back into the glass and that should be covered or the weave will show through the paint.

When doing the outside its much the same process, rounding off the ply “corner” first, and if you’ve used plastic ties or lacing you can plane it off otherwise you’ll need to pull all of the wire. Note that if the wire does not want to come you can heat it with a heavy soldering iron or blowtorch and the epoxy will let go.

Next, fill the gaps and holes, and away you go wet on wet with the resin and the tape, when hard then sand the roughies off and trowel on that wide fillet of low density filler and sand off.

If you make a tidy job, the tapes will not show, and it’s a trademark of a professional job that all these joints will look as though they grew together rather than being joined by someone as ordinary as a home boatbuilder. Its not hard.

*A note on sandpapers, and perhaps I should visit this in the future sometime as it is a very much more complex subject than most realise. But in the case where sanding cured epoxy feels like trying to wear away solid steel with a wet towel, something better than the garnet or cheaper alox grits that you get from the local hardware supermarket is indicated. If you ring a car body repair shop and ask who supplies them you will get a supplier with a range of high quality flexible abrasives (the stuff we call sandpaper) at a price that is a lot better than you might think.

Remember that “sandpaper” is a tool, and a tool that may be very specific to a particular job so you need to buy the appropriate type and grit. It should be stored in an airtight container with some silica gel to keep it dry and crisp, and store it in such a way as to prevent the grit surfaces from being in contact with anything that might take the edge off.

In the case of sanding epoxy I use an Open Coat Silicone Carbide 60 grit cloth backed “paper” of a type used in commercial panel sanders but this can be hard to come by unless you have a Plywood or Particle Board mill nearby. I’d suggest a “Name Brand“ of Aluminium Oxide (blue or yellow) open coat 60 grit on a heavy duty backing if you cant get the sort of thing I use.