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28. Coating kayaks with high performance two part polyurethane (Interthane Plus)

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This polyurethane seems to be a pretty obscure item. Many marine stores do not carry it. I got it from Defender which doesn't even list it in their huge catalogue. Make sure to ask for the 'clear' type since this line contains mostly pigmented coatings.

Interthane Plus (a two part polyurethane, which has been renamed to "Perfection Varnish") is another top notch product which will protect the epoxy from UV damage and give your kayak the ultimate gloss and a scratch resistant coat. Unlike many varnishes, it is completely clear (it has no hue or tint). It is also substantially more expensive ~ $50/ quart or liter. Add the 'must have' solvent and it comes to $60+.
It works like a normal varnish but it also has epoxy-like properties. A normal varnish or single part polyurethanes are essentially a mix of solid resins suspended or dissolved in various solvents which improve flowout, leveling of brush marks, bubble dissipation as well as working viscosity. When the volatile solvents evaporate, the solid resin remains and hardens by slow air curing process. You can smell varnish coat after many months, which indicates that there are still volatile molecules degassing from the coat. The resin and hardener in the Interthane Plus, on the other hand, react together to form a solid which is superior to varnish (or other single component systems) in all desirable mechanical properties. This includes hardness, scratch resistance, solvent resistance and surface quality, namely gloss. The solvent added to Interthane acts in the same way as in varnish. It lowers viscosity, flowout etc...After couple days, however, there is no discernible odor from the reducer and the coat feels and looks like solid glass.

I use the small 4oz. disposable cups to dispense the measured quantities. You got about 2 minute window to measure off the amounts and pour it into a polyethylene or some other mixing container. If you wait longer the styrene cups disintegrate right in front of your eyes. I dispense the larger portion of the resin first, then reuse the cup to measure off the hardener with the solvent together. The solvent thins the dense hardener which can be then poured out with little waste.
Mixing:
Interthane Plus is mixed in 3:1 ratio by volume (3 parts resin, 1 part hardener). I thin it further by adding 1 part of the 2333N reducer to achieve 'my ideal' viscosity for foam brush application. Make sure to use the 2333N ONLY!! Other solvents will not work. The oil based Interlux varnish brushing liquid 333 will definitely not work.

The entire package was sufficient to give the kayak 4 coats total (4 hull and 4 deck) with little leftover.
  Interthane Plus contains some very potent solvents. I always pour a test sample on my 'chemically inert' plastic table but in this case, the poly has fused in irreversibly and softened the plastic to a considerable depth (whereas varnish or epoxy would just set and peel off). The hardened Poly film is very strong and is similar to cellophane (clear hard candy wrappers). By contrast, varnish never really hardens but always remains soft and ductile (stretchable).
NOTE: Interthane Plus must be applied in thin coats not only to prevent drips but because the solvent must evaporate to a large degree from the mix so that the two components can properly react and harden. The timing seems to be all important. If you apply a thick coat (like the bar top epoxy) or leave it in the container, the poly will gel, turn opaque but will not harden - ever. This is consistent with the behavior of thinned epoxy. The reducer simply interferes with the chemical reaction and if the polymerization is not completed in a set time the coat is ruined.
The white puck (leftover gelled batch) is a good example of this. Also, the solvent which was completely blended in actually exuded (sweated out) out of the mass.
The brush application is no different from varnish. Interthane will also sag and drip when brushed on too thickly but the 'window of opportunity' which allows you to blend or feather older and new coats together is slightly longer than for varnish. One great advantage is that Interthane can be recoated without prior sanding. This is particularly useful on the first coats. Recoating without sanding of course leaves the old dust on the previous layer. It has has to be eventually sanded off prior to the finish coats. The sag peaks will be highlighted with a magic marker (see previous page) and knocked down with wet sandpaper (220 or 180 grit).
 
S&G Cirrus
You will hardly get a better gloss with anything else.

Interthane Plus Pros':
  • Scratch resistance - if you run a piece of metal over varnish, it will always scratch or scuff. Interthane will not. It can withstand scuffs and minor abrasions without a mark. Wet sanding, which can remove a whole varnish coat very quickly is far less likely to cut into the poly as aggressively. Consequently, poly builds up thickness in fewer coats than varnish.
  • Chemical resistance - If you wipe varnish with a strong solvent like a lacquer thinner, the surface dissolves and the varnish loses gloss. Interthane is completely immune to that. It can stand to be bathed in thinners without ill effect (astonishing!).
  • Gloss - is superior to varnish without a question. It's almost too much since it will highlight the flaws one would rather not see :)

Interthane Plus Cons':

  • I really hope that the 'New and Improved' version of this product will come with the hardener packaged in a screw top container and little thinner. The thick syrupy hardener is a pain to dispense in small quantities and with every tilt of the can, large amounts of the precious liquid becomes trapped in the deep rim of the container. Here, it foams up and hardens around the lid like acrylic glue. Even a short exposure to air, causes the hardener to skin over, which is not the case with the resin. Who would really care at varnish prices but at $50 a quart and having little flexibility with the tiny amount supplied, every drop really counts.
    Despite my best efforts to be clean and efficient, I run out of the hardener but still had enough resin for a solid coat and a half. So, Interlux, if you ever read this, show us mercy and put the hardener in a screw bottle, thin it with 1/2 part of thinner so that the new mixing ratio becomes 2:1. No one can use it anyway without at least this much thinner added in.
  • Price - is stiff but it's worth it.
Protective plastic "roof" is very helpful if wood dust keeps raining from overhead light fixtures or rafters.
Four feet into polyurethaning.
One half of the deck is done, the other half is feathered in on the deck center line seam.

The coaming can be done first or last. Here, I am doing it last.

Just finished the rear deck on the plywood Cirrus. The brush marks are still visible in the reflection. The pot life on the Interthane is quoted as 8 hours so the coat has enough time to level out. Only 3" masking tape is used to mask off the already finished hull.
The hatches can be varnished at any time. I use whatever is left over in the cup after doing the kayak and coat the small items like rudder, hatches, bulkheads, composite handles and hardware, backrest.........

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Last page update: 29 October 2013