Bear in mind that whatever advice I offer here is my opinion, based on my experience. If you disagree, that's fair, but I'm not writing here to give equal time to opposing points of view, which may be just as valid, by your lights.
A common mistake novices make is not thinning whatever varnish they're using. Often, the instructions on the can will tell you not to thin, in bold-faced letters, or to thin no more than 5%. Rest assured that it is a manufacturer's disclaimer, intended to prevent the user from altering the chemistry of the product, which, of course, they cannot be liable for.
It's almost impossible to get a level mirror-like finish with no brushmarks or lap lines, without thinning. Contrary to label instructions, I thin varnish 30%, even 40% of volume, for every coat, according to weather conditions. Be forewarned, this will make a thin, runny mix, but it will dry fast, and that's how you can do multiple coats in a day.
Another reason to thin this much is that in a subtropical climate like Florida's, varnishing in the sun is unavoidable for most – again, contrary to manufacturers' disclaimers.
If you apply unthinned varnish to teak in the hot sun (remember, it can be 90° Fahrenheit or 32° C by 9 o'clock in the morning in Florida), the varnish will wrinkle, or 'alligator', disastrously, because the top of the liquid will dry faster than underneath, and shrivel. Thinning prevents this action, but it's best to avoid laying varnish from two hours before noon till two hours after it in the summer, unless you're in Sweden.
I've done some varnishing in one of the big sheds at a yard, and there are advantages to that – you don't have to worry about wind, sun, or rain. But I'd rather not work on a boat indoors. I do this kind of work because I love boats kept Bristol fashion, the sea, and fresh air.
It is good practice to use the maker's recommended thinner, or 'reducer', and with two-part finishes, it's critical to do so. In that case I would use the proportion called for in the instructions. However, with traditional spar varnishes, for everything from Schooner's to Epifanes, I've found that plain old paint thinner works fine with every one I've tried.
Be sure to use a product labeled 'paint thinner', rather than 'mineral spirits', as simple paint thinner is not as highly refined as spirits, and will dry slower, which is the whole point of using a thinner (unless it's cooler weather, and you want it to dry faster).
Since waterways act as wind tunnels, there's almost always some wind at a marina. The windier it is, the more thinner I add, and if it's blowing fifteen knots or more, I may have to add a couple capfuls of thinner every fifteen minutes or so. Wind and sun work against the varnisher, though a light breeze is good to keep the no-see-ums at bay.
Here in Florida, when the seabreeze picks up in early afternoon, the moisture in the sea-air may cause the varnish to 'stand up', and not level as well, or even to form little air bubbles. I then add an extra capful of thinner to my varnish pot to compensate.
Bubbling in a dried finish may be caused by residual moisture trapped in the wood. That's another benefit of using a heat gun to strip old finishes – the heat helps drive such moisture out of the wood.
Some people have the misconception that all varnishes dry slowly, which is a holdover from the days before metallic salts were added as driers. A varnish that contains a high proportion of tung oil, like Epifanes or Flagship, will dry slower than others, but this should only be of concern if the ambient temperature is less than 70° Fahrenheit ( 21° C), and then you shouldn't apply it too late in the day.
I'm no fan of water-based finishes. They may have some ecological advantages, but they aren't as durable as oil-based varnishes. Like polyurethane, they're quite sensitive to atmospheric humidity, and will 'blush', or cloud if humidity is high. Even oil-based varnishes will do this if applied too late in the day and the dew hits the finish.
Also, remember that humidity is highest just before the dew point is reached when the air is saturated in the evening (usually right before sundown) or the early morning. Humidity can be a factor even indoors in a non-climate-controlled environment like a garage or workshop.
Finally, geographical location does make a difference in application methods – most of my varnishing experience has been in Florida, so keep that in mind and adjust your practices according to local conditions.
