I've messed up so many varnish jobs that I'm somewhat hesitant to pontificate on the matter. But, from every huge sag, every dust-filled "non-skid" finish, every coat that didn't dry, I learned something. In fact, I can claim a small, but growing, portfolio of perfect varnish jobs. And it is the following three steps that are most responsible for my varnishing success.
Prepare Surface: These are the two most loaded words printed on a varnish can, but the manufacturers really could have stated it with just one word: SAND. Varnish won't hide anything, so early on I learned that no amount of sanding is enough. This really is the simplest step towards a good finish, but it's amazing how many builders skimp on this easy, if monotonous, task. After you've applied two or three coats of epoxy to seal out moisture (wood can be waterstained even under multiple coats of varnish), sand first with 120 grit, then 220, until your surface is a uniform opaque - no little glossy indentations in the epoxy. Clean the sanded surface with a solvent - mineral spirits, lacquer thinner, even water, but allow more time than you'd think for it to evaporate. Varnish won't dry with even a trace of that stuff under it.
Varnish In A Dust-Free Workplace: This is an unlikely suggestion from a guy who hasn't dusted his apartment since, well, never. But you'll find the area where I do my varnishing sterile and absolutely dust free, given the apparent magnetic attraction of dust to wet varnish. If possible, soak the floor all around the boat with water just before you commence varnishing. Don't forget that if you've been in the shop all day there will be dust on your clothes, on your respirator, and in your hair. Some pros varnish shirtless, but discretion is advised.
Use Proper Brush Technique: My grandfather was proud of having maintained the same badger-hair brushes for 30 years by using elaborate clean-up and storage methods, but as someone who can't clean up his desk, I advocate disposable foam brushes. Foam brushes are the best tool for applying a thin, even coat. When varnishing it's critical to maintain a "wet edge." Make slow strokes in one direction only, never back and forth. And always start the loaded brush on a dry surface, then move into the wet surface. Sags are caused by a combination of deep brush strokes and too much varnish; use a light touch and err on the thin side. Marine varnishes get tacky quickly, so resist with all your fiber the urge to go back and fix holidays and sags - you'll just make it worse. Since you'll be putting on three to five coats, you'll have the opportunity to cover bad spots. I wet sand with 400 grit between coats to maximize smoothness and to avoid sanding off too much of the previous coat.
A Word On Varnish: Avoid home-center varnishes, often labeled "spar"" varnish. They seldom have the UV filters to give lasting protection to the epoxy. Any of the recognized marine varnishes will do, such as Interlux, ZSpar, Epiphanes, or Woolsey. At CLC, we use Z-Spar's Captain's Varnish for its golden hue, hard finish, and easy application.
John is shop manager for Chesapeake Light Crafts, purveyors of kayak kits and most things used to build wooden craft. John's work number is (410) 267-0137.
More on Varnishing
When I called Boat-US to order the Interthane Plus for the hull, they read me their informational warning that said that this finish should only be used above the waterline. Since that's a recommended finish in the Windmill manual, I would assume it's OK to use on my boat, which will be dry-sailed. It goes for around $50 a quart these days and I'll probably need two quarts to put on two coats, so I just need some reassurance.
When they talk about "above the waterline," they really mean to say that it's not to be used below the waterline if your boat lives year-round in the water. I don't understand why they don't make that distinction but I sure get a lot of calls about it.
Interthane Plus is an amazing finish: amazing in beauty, expense, and sheer torment in application. I understand that a lot of boat dealers put this stuff on OVER THE GELCOAT of new boats before big boatshows just to make them look shinier. It works. My Windmill has Interthane on the topsides and interior. It looks great but it gave me fits when I put it on. I think that it's really meant to be sprayed, and in a humidity-controlled booth at that.
I would encourage you to use it, but you might want to do some large-scale testing first, to get comfortable with the cocktail of "special reducers" and thinners and pigments and catalyst and whatever else. The first time I used Interthane I did just about everything wrong: it was too hot and humid, I didn't get enough "reducer" into it, I had the wrong kind of roller and brush (they fell apart in mid-coat), and I didn't let the solvent with which I wiped down the boat evaporate thoroughly before I started painting, which led to total failure of the paint in a number of places. Watch for those things. I would highly recommend using a primer of some sort, for two reasons: A) it helps fair the hull (tiny imperfections will be very loud beneath such a glossy paintjob), and B) primers seem to stick better to epoxy than the high-end paint finishes, which wither in the presence of a single amine molecule. An epoxy-primer barrier coat would be good because it's a strong undercoat, compared to the somewhat chalky high-build primers I use to get fru-fru finishes on some of the boats I do for show.
John C. Harris, Kent Island, Maryland "John's Cool Boats"
Still More on Varnishing
I have used Z-spar Captain's Varnish with a lot of success but I did not like their waterbased varnish at all.
I really appreciate your getting back to me so quickly. I'm hoping for some additional responses, but your negative vote counts a lot. I'd like to know what you found unsatisfactory, because I finished the forward thwart seat for my boat with the water-based Z-Spar and it turned out pretty nicely.
CLC has been selling Z-Spar Captain's Varnish for years; I've probably put it on twenty boats. I wasn't aware there was a water-based variant of Captain's Varnish, never used it. There are several varnishes I like, though since it's what's on the shelf I always use Captain's Varnish. It has the right viscosity on the brush, doesn't go on too thick, and has a bit of a golden hue to it that seems to enhance just about any wood grain.
John C. Harris