Tape Sealing Outside Seams [41]

By David Winkworth

After rudders and feathered versus un-feathered paddles, tape sealing of outside seams is probably one of the most controversial subjects in sea kayaking.

Does it add strength and improve sealing? Is it worth the time and effort? As surely as the sun rises in the East, there will always be disagreement on this topic.

My cards have been on the table on this issue for some time. I tape seal every boat I make. I think it is worth doing, I think it strengthens the join and confirms water-tightness. I also think (know) it is fiddly to do, time-consuming to do a neat and thorough job (the only sort of job worth doing), and relatively expensive.

Larry Gray (NSW Sea Kayaker — Issue 40) said that if he thought tape sealing was worth doing, he would instruct his builders to do so and add $20 to the cost of a boat. Maybe you get what you pay for in this life; I wouldn’t tape seal a boat for less than $150.

The alternative to tape sealing the outside seam in conventional glass fibre sea kayaks is to mask up the join and apply gel coat or flowcoat (gelcoat with wax-in-styrene added). Problem is, I believe, that gelcoat doesn’t add any strength — it is purely a relatively brittle sealing surface. Therefore the inside seam in such a join has extra work to perform. It must work in both compression and tension mode at the same time. If that seam allows the join to flex, the gelcoat strip can crack and allow water into the fibres of the hull and deck laminates or into the compartment.

Tape sealing the outside seam adds another seam to the join so that this inherently weak butt join of the hull and deck is held from BOTH sides. The two seams are separated by the width of the hull and deck laminates, and can work independently and more efficiently in tension and compression modes in the event of an impact. I’m talking about sea boats here — despite our best efforts we do occasionally hit the bottom (or each other!).

The one problem with tape sealing of outside seams, as Larry correctly points out, is that the outside seam laminate is applied to cured gelcoat whereas the inside work is done on ‘green’ or uncured glass/resin in the mold. There is no doubt that the inside-hull bond is the stronger of the two. It a chemical versus physical bond. However, with thorough sanding and deep scoring of the gelcoat on the join, I believe effective adhesion is achieved. Preparation is the key. Like I said, it’s not cheap! This outside adhesion is no different to any glassing-in of bulkheads, etc inside the boat a few days after the lay-up is complete. By this time, the resin in the boat has cured and the bond is physical. We trust our bulkheads don’t we?

So, to tape seal or not? Well, I suppose it depends on the sort of work you do with your boat, whether or not you have had any seam cracking and your inclination to get dirty!

The following description of tape sealing is how I do the job. It is certainly not the only way it is done and it is not meant to be a definitive work on the topic. I can offer no guarantee on it and I suggest you talk it over with another builder who also does tape sealing. I think the job requires some knowledge of glass fibre and resins. If in doubt, get some help.

OK, here we go… remember, this is my procedure only — it may not be the one for you, but feel free to call me if you have any questions.

The boat must be clean, salt-free and dry. I support the boat firmly on edge and do half the boat at a time for each procedure. I wear protective glasses, latex gloves and an effective double filter mask.

Using a fine 100 mm sanding disc in an angle grinder, I sand off the hull and deck edges until they are smooth no more than a few millimetres either side of the join. Be careful.

Using 25 mm wide masking tape (use good quality tape only), I cover the join evenly and smoothly. Look along the tape. If this piece has waves in it, so will your join when complete. Now add another strip of tape above and below the first with a gap each side of about one to two millimetres. Go just around the bow and stern. Now remove the centre piece of tape and you have the area you need to sand.

I use a coarse Speedfile paper — about 40 grit. I work along the strip right up to the masking tape — no shiny gel coat should be visible. In some places the masking tape will rip a bit. Just add another little piece over the rip later. Don’t be afraid to cut right through the gel coat if you need to in order to get a smooth strip. In some places, you may need to fill the gap with a filler such as a Q cell mix, Probond or similar. Vacuum up the dust and check for missed bits. Now I wipe the join thoroughly with an acetone soaked CLEAN rag and allow to dry.

I then lay out the 25 mm glass fibre tape. I use little bits of plastic to hold it near the join. Do not put masking tape on it — you will pull the fibres out of shape when removing it. Using waxed resin (so it can be sanded after a few hours) I apply the tape with a brushing motion. Using too much resin will have it pooling under the tape. When tape is on, remove the masking tape before the resin gels. When resin has set you can do the other side of the boat similarly.

With kayak still on edge, mask up the taped area again on both sides of it with the tape edge again about 1-2 mm out. Using finer paper (I use the green stuff which comes on a roll — fine or medium grade), sand the glass tape well, also getting an edge of the sand paper into the strip between the glass tape and the masking tape. Wipe well with acetone and allow to dry.

Then I use coloured BRUSH flowcoat. Spray flowcoat is too thin and you won’t get good opacity in the finished job. I apply it smoothly with a brush, and then go back over it all with a piece of open cell foam in a stippling motion, adding flowcoat to the foam when needed. Gelcoat/flowcoat is thick and brush marks will not subside as they do with paints. I find the seamless stippled finish preferable to a zillion brush marks! Pull off the masking tape before the flowcoat gels. Clean up any spots of flowcoat with an acetone rag.

So that’s it? Well, no it’s not. I find that another similar coat of flowcoat is necessary to get the join nice and smooth but that’s up to you. If you go through it again, be sure to sand the flowcoat well and wipe with an acetone rag to remove all traces of wax.

Now you can do the other half of the boat! Well, I did say it was time consuming!