How to Varnish boats

Varnishing is almost a black art to the beginner faced with a boat. Awkwardly shaped and not just a square box. The challange becomes one of how to cover those awkward bits where varnish will try to puddle, run or be difficult to reach at all. Then there are the edges, where the brush should end up at, and not begin its stroke. This first word of advice is a clue to several other techniques that should be observed. The contrary point is that an inside corner is where the brush should begin its stroke and the varnish worked away towards the open and flatter areas.
The brush should be held lightly and allowed to trail at its edge and there are two distinct stages of first putting down, and secondly, of spreading out as evenly as possible onto the surface as speedily as possible, consistent with being unhurried and not slapdash.
Most of the time thinners must be added. A first priming coat to bare wood needs to be applied with somewhere between 25% and 50% thinned.
Varnish is generally too thick to be applied without any thinning at all. So I apply all intermediate coats with about 15% thinners added. The final coat is best applied 25% thinned.
A patch of about 2ft X 3ft should be worked until there are no thickly covered areas likely to run and cause dribbles. The brush should carry no more than 3/4 of an inch dipped into the pot and applied boldly in strips; these widened from about 3 inches to 5 or 6 inches. The next strip is positioned parallel about three inches away so that a gap of uncovered surface separates the two strips.  The brush is then worked at right angles to the strips joining them up, spreading and avoiding the chance of applying too much. As soon as one patch is covered, move along to the next, blending in as much as possible. Only after some practice will this become automatic and second nature. Done correctly this method should allow one to cover greatest area in the shortest time.

Immediately after application, the varnish will begin to get sticky and  more difficult to spread evenly. The good news though, is that initially, what looks striped with brush marks, will flow out as the varnish settles itself onto the surface.

After approximately 8 coats the surface is still not even

Nice colour without any stain added

The varnish is still slightly tacky at this stage but will improve slightly once fully dry

Over many years of practice I have experimented with some novel ways of getting that elusive "grand piano" surface. Dust is the enemy. Preperation is key. Vacuum the surface, wipe with "tack rag",  or simply brush across the surface with the bare hand, and the dust will be felt for certain. Get rid of much as possible - every little speck. While getting a new dinghy looking its best I have been known to suspend it inverted from my garage roof and varnish from below in order to try for that dust free finish.
After an area is completed it should be looked over carefully to spot any runs or dry patches. Runs can sometimes be corrected with a moderately dry brush or if the drying has made the surface too sticky - left till properly dry before sanding off with wet/dry paper. Beneath the run there is usually a residue of sticky varnish. Remove it by applying thinners on a dampened rag. Allow to dry for several hours before further rubbing down to a matt surface. 

After each coat is fully dry - usually 36 hours is enough. Rub down with a medium wet and dry paper About 230 grade is ok. Put a drop or two of wash liquid into the bowl of warm water and if the hands are skinned by the paper wear plastic or rubber gloves to protect your skin. Take off all the shine so as to produce a milky slurry, wipe clean with a sponge and finally tack rag dry with some alcohol spirit applied sparingly. As suggested already dust off  properly before applying the next coat. 

Varnish is fairly expensive but spending a little extra for better quality is best policy. Tung Oil is often found in best quality varnish, helping the flow properties.

Much of what is suggested here will hold good for single pack  Polyeurathane or twin pack Epoxy varnish, but the latter cures by chemical reaction, not air drying. A thicker coat can therefore be built up a little quicker. I know proper boat builders will apply several Epoxy coats, soon after each undercoat is dry. Only then is the wet and dry sanding begun to flat down the surface for the final two or three coats.  The big benefit of Epoxy is its durability, as a carefully looked after craft will still look good after five years - conventional single pack varnishes - not so long.

How many coats of varnish in total?  Eight will do but for the best finish probably double this number!  

There is of course always the Rowan Atkinson Method (RAM) where Mr Bean applies just one coat!....Fabricate one improvised explosive device (IED) and place it beside a paper cup of varnish. Light the blue touchpaper (BT) and quickly retire to a place of safety (POS). 

Notice that even in this extreme example, preparation is key to succcess.