The Sea Witch's Brew - by an unknown source

One of the many experts that I talked to for their recommendations on this issue pointed out that 50 minutes of every hour re-varnishing should be spent sanding. The remaining 10 minutes is for actually applying the varnish. If you don't heed that advice, your project is doomed from the start.
There are three common types of varnishes found in the marine world: alkyds, polyurethane and phenol. The alkyds are the least expensive but aren't as tough as the others, so your varnish choices will probably be a urethane or phenol. The urethanes, which include the two-part mixtures, are notable for their resistance to abrasion and long-term gloss retention, although they are relatively brittle. The phenol are the traditional varnish, often called spar varnish, with excellent flexibility that allows them to stand up to bending or blows, but it makes them somewhat less resistant to abrasion. Bear in mind that most varnishes have ultra violet protection against the sun's rays, which can add a golden glow to the color of the varnish. That adds a luster to most woods, but it makes a pale wood such as ash turn yellowish. 

If you're working with new wood, or with old wood that has been stripped and then stained, you'll need to start with a sealer coat. This can either be one of the commercial sealers or it can be a thinned version of the varnish that you plan to use for later coats. In either case, you'll probably find that it raises the grain of the wood slightly, which allows it to adhere tightly. Some pros don't sand between the sealer and the first coat, reasoning that the roughness gives a better grip and will be removed in later layers, although most manufacturers suggest a light sanding to knock off the stubble. 

For sanding, the choice of most professionals is the so-called "production paper," a brownish sheet inexpensive sandpaper. You'll need a wide range of grits, although 220 is probably the most common for roughing up previous coats of varnish. Don't save your sandpaper ... throw it away as soon as it starts to look worn, because you'll just be moving the same dust around otherwise. 

If you don't subscribe to Harrah's educated thumb method, an orbital sander can ease your workload immensely. Most pros use the so-called "quarter sheeter," which is compact and accepts a quarter-sheet of sandpaper for minimum waste. These often accept a "stack-load" of several sheets of paper so you can simply tear off the old paper without having to stop and reload. If you must hand sand because of awkward or tight spaces, then you should use a compact sanding block to keep from creating an uneven surface. Even though it sounds obvious, there are still people who don't know that you have to sand with the grain so, whichever sanding method you choose, be careful not to cause scratches that will show clearly under the varnish. 

After you've sanded your surface, bring out your vacuum cleaner and suck up all the dust from the varnished area as well as the surrounding surfaces. 
You can also dust off the varnished area with a stiff brush, and many wood craftsmen then wipe down the sanded varnish with either paint thinner or alcohol on a clean rag. Finish up your dust removal with a tack rag that picks up any stray dust particles. 
Before you touch the can of varnish, however, you should discard every idea you have about painting, because varnish is a totally different creature. 

James Bond may have liked his martinis stirred, not shaken, but varnish should never be stirred or shaken. The only exception is some satin varnishes that need a gentle stirring to keep the dulling agents in solution. 

After opening the varnish can, pour a small amount of varnish gently into a paper paint bucket through a disposable strainer, keeping bubbles to a minimum. 
A good brush is essential for a good finish, and you should never use a varnish brush for paint, unless you like the idea of tiny flecks of paint in every future varnish job! Dip the brush into the varnish only about halfway up the bristles but, unlike paint, don't remove the excess by wiping the bristles on the edge of the bucket, which will cause bubbles. Simply hold the brush for a moment to drain, and then, holding the brush near the base of the handle, flow the varnish onto the surface. Nearly every pro refers to the motion as a "pulling" stroke, with the emphasis on smoothness.   Holding the brush in a fairly upright position, use long strokes to work sideways across your surface. At the beginning of the stroke, there will be more varnish deposited so, by overlapping your strokes as you move along the surface, you even out the coating into a smooth finish. 

Varnishing is, to some extent at least, a black art and everyone has to find his or her own style and method. If the varnish is too thick, it will result is obvious brush marks and overlap streaks. On the other hand, thinning the varnish too much can cause sags, runs, and a thin coat that will require many layers to build into a suitable finish. 

"Wet edge" is a varnishing term meaning the ability of the varnish previously done to accept another stroke without sticking or dragging into a smear. Here the pros have some disagreement, since some like to thin their varnish with solvent, while others prefer to work quickly before the varnish becomes tacky. If you find that you've lost the wet edge as you fair or "cut back" the new brush stroke into the previously varnished area, you'd better do one or the other to save the finish. 

Always plan the varnish project ahead, so you have natural stopping places such as the edge of a surface, a joint, or a fitting that hides the brush marks where you start or stop. If it will be difficult to "cut in" the edge around your varnishing project and the surrounding paint or fiberglass, be sure to tape off the edges. Don't ever use masking tape, however, which is scornfully called "something that house painters use" by marine pros. Besides not creating a sharp edge, it can also remove the underlying finish and, if not removed promptly, can become a permanent part of your boat overnight. An industry standard seems to be 3M's "Fine Line" tape for easy removal and a sharp edge. Be sure to burnish it down with your fingernail before starting to varnish. 

How many coats do you need? Obviously, the more coats you have, the better the wood is protected, but you can also overload the surface and create a rubbery finish. Six to eight coats was the most common range, although many said that it depends on the wood. When you reach the point where the wood grain is no longer visible and your sanding produces a smooth hazing without grain streaks, then two more coats are sufficient. 

Edges of the wood, such as corners of a hatch or a cap rail where you regularly board, should get extra layers of varnish for protection. These should go on in the middle of your varnishing sequence, and should be faired in smoothly during later coats. 

When you've finished your project, throw away any leftover varnish in your bucket rather than pouring it back, since it now carries dust that would contaminate your fresh varnish. Seal the varnish can tightly and, if you're going to store it, exhale into it before you put the lid on to displaces the oxygen, which can cause a skim, with carbon dioxide. 

It's almost certain that you'll nick or ding your varnish during the season, usually just after you've refinished, so you should be prepared to touch up small spots to keep the wood underneath from discoloring from water contact. A bottle of finger nail polish, emptied and rinsed with acetone, can be refilled with varnish as a handy touch-up tool, and you're more likely to use it than bother to dig out the can of varnish and a brush for a tiny flaw. 
When sanding between coats, you should protect yourself with dust mask, goggles and protective clothing such as coveralls and hats. But don't forget to shake out those same clothes before varnishing, or you'll carry your own dust storm with you. 

If you're doing extensive varnishing, it's a good idea to stop and clean the brush every two or three hours, or when it starts getting stiff from varnish build-up. Brush cleaning is as much an art as actual varnishing, but you'll want to use clean solvent, flex the bristles to loosen dried varnish, and then repeat the process with fresh solvent. Most pros dry their brush with a spin dryer, available at paint stores for a few dollars, and some even use bristle combs to straighten the bristles. If you're going to be using the brush on the following day, you can lightly clean the brush and then store it overnight in clean solvent, making sure that it is suspended so that the bristles don't touch the bottom and warp. 

Sunlight and weather are the enemies of varnish, so you'll need to protect your finish as much as possible. Sunlight can heat the finish, causing the wood to contract and cracks to appear in the finish at joints, so covers for your varnished surfaces are particularly important in warm areas. Saltwater that is allowed to dry on varnish leaves crystal deposits that act as tiny magnifying glasses for sunlight, so rinsing the varnish regularly is essential. Water is not so much an enemy of varnish as to the appearance of the surface if it is allowed to penetrate and darken the wood underneath, so touches up dings promptly.
Teak, because it is an innately oily wood, requires a slightly different approach according to most yacht refinishers. After sanding the bare wood with #80 grit (slightly coarser than for other woods), you should wipe the surface thoroughly with paint thinner, turpentine, or one of the special solvents such as International 216, to remove as much surface oil as possible. The first coat of varnish should be thinned to get as much penetration as possible into the wood both to seal the oils in and to provide a solid anchor for future layers. From that point, the varnishing continues until the desired finish is achieved. 

One way to achieve both depth and toughness on all woods is to combine phenol varnishes with a two-part urethane such as U.S. Paint's Awl-Brite, which can be applied over a conventional varnished surface for a hard outer layer. Awl-Brite is brush able, but it can be tricky and the directions must be followed exactly. The previous varnish must be in good condition and at least two months old so that it is fully cured, or the Awl-Brite solvents may create wrinkles and blisters in the old finish. It is more difficult to keep a wet edge, and it must be applied and dried in shade. It is also somewhat thinner than conventional varnishes, so care must be taken to prevent curtains and sags on vertical surfaces. On the positive side, two to three coats of Awl-Brite can be applied in a day because it dries quickly. 

While it won't make you happy, varnish provides a tell-tale clue to every mistake you might have made. "Blooming", for example, is the whitish appearance caused by the absorption of moisture while drying, which is why you shouldn't have varnished late in the day before a dewy night. Alligatoring (called crocodiling in England) is a pattern of deep cracks caused by applying a new coat of varnish before the previous layer has fully dried, while chipping or flaking is simply the way old varnish says you should have added a coat or two in mid-season before it was too late. Blistering is caused by moisture in the wood that vaporizes and lifts the varnish film. A patch of wrinkles means that you tried to varnish in hot weather, or you applied too much varnish that never dried properly. 

It all sounds difficult, but varnishing is really a pleasant skill of the seaman. It provides more exercise than a round of golf, and a cold beer is usually nearby. Of course, beautifully varnished bright work is an ego trip, because it doesn't stand up as well even the lowliest paint and it refuses to hide any flaws under its surface. 

Nevertheless, the sparkle of a fresh coat of varnish is its own reward, and the satisfaction of running your hand along a satiny smooth section of varnish makes all the effort worthwhile.