Most any Spar Varnish will work, Z-spar Captains seems to be the most widely used brand, it costs around $20 for a quart which is good for 4 to 5 coats on your standard sea kayaks.
The main purpose of varnish on your wood boat is to protect the epoxy from ultraviolet damage while you are out in the sun. Check to make sure the varnish you choose has UV protection.
There are several techniques for applying varnish to your boat. Most people use foam paint brushes available at most hardware stores, some use a clean rag, some spray it on with high-tech, very expensive equipment.
Creating a dust free environment to varnish in will help you achieve that mirror finish you want for your wooden beauty. Sweep and clean the day before you varnish then wet the floor down to help keep dust from being kicked back up into the air. You can also use an ionizer to knock the dust out of the air and help keep the room dust free as the varnish dries. Start it up an hour before you want to start varnishing.
Apply the first coat and let it dry 24 hrs, flip the boat over and do the other side, let dry another 24 hrs. Lightly sand the entire boat with 220 grit or finer sandpaper, clean the boat off with water and a clean cloth, let it dry thoroughly and do another coat.
Quit varnishing after 3 or 4 coats and go PADDLE!!! after all, you built it to paddle, not to look at!
Others have found that polyurethane varnishes have worked well. Helmsman by Minwax has been used by a number of builders with good success.
Technique for using a clean rag to varnish a kayak:
- Do a reasonable job of sanding the epoxy to the finish you want. Varnish will not do much to change surface imperfections.
- Use a tack coth (clean cloth which has a little half dried varnish in it) to remove the dust
- Use a clean cloth to wipe a thin coat of varnish over the whole boat. The idea is to apply a sheen of varnish, not as thick as a coat of paint. After several coats, go back and use some fine grit sandpaper and a tack cloth again, before finishing up with several thinner coats.
Rob Macks has described success with varnishing outdoors in keeping dust off the varnished surface.
— Another option for a protective finish coat:
You can also use a two-part linear polyurethane coating from System Three called WR-LPU. It is a water-based product that dries to an extremely hard chemical resistant coating. You get the hardest coating if you use the cross-linker on the top couple coats. It dries to the touch very quickly but takes a week or so to fully cure.
Application is much like varnish except you need to apply it in very humid conditions because it dries fast and it needs enough time to smooth out. It can be cut with water to help extend the drying time. It sets up fast enough that you can get 5 or 6 coats on in a day (the recommended min. number of coats is 3-4). The nicest thing about WR-LPU is that it doesn’t turn yellow like varnish does. One of my kayaks is in its 3rd year and the wood is the same color as the day the first coat was applied. It also has good UV protection. It comes in glossy and flat. See SystemThree.com for more information.
For Skin on frame Kayaks
One-part Polyurethanes
Polyurethane varnishes rule the roost. Saturating cloth with a first coat takes the most varnish. Follow on coats take a little less. Many builders use between 8-14 coats of varnish.
Polyurethane varnish comes in a oil-based version and a water borne version. Either works for SOF boats. Some of the solvent based varnishes have UV protection. However, some users have noted that some oil-based polyurethane can be brittle and poking the fabric may lead to concentric cracks – a bull’s eye. Waterborne polyurethane also works well for many builders, though the packaging of these products almost invariably state it is not intended for use outdoors. Waterborne poly is much clearer than the yellow tinted oil-based poly products. More SOF builders are moving away from water-borne formulations, though. They are not as durable as the oil-based. The best oil-based formulations are fairly flexible and don’t have the cracking problems. Zar™ is an often-recommended brand.
For all polyurethane based varnishes, it is very important to follow the directions for minimum and maximum times between coats, and for the curing time necessary after the last coat. Typical times for water borne varnish are timing of the coats between 2-6 hours apart, and a final cure time of 3 days. Occasional builders have noted that especially waterborne polyurethane which is not allowed to cure may peel off after use.
Two-part Polyurethanes
Two-part polyurethane varnishes are the toughest and among the most expensive coatings for SOF kayaks. However, many builders swear by the results. Two-part formulations are harder to find and can cost on the order of US$200/gallon. The mixed polyurethane has to be applied in a well ventilated environment and you must wear a respirator rated for chemicals; a dust mask is not acceptable.
