I've been waiting to see if you got any answers.

I'm curious myself about this. I spent some time on the Internet looking for answers, and I talked to a friend in the auto finishing business who is my go-to expert on all things paint and varnish related. I found this part of a FAQ on the West Marine site that seems to hit the nail on the head.

"When to varnish

Avoid the “I have to do it today” syndrome: Be patient. Wait for the weather to be right. Sand thoroughly. Do all the prep work. This dreaded syndrome is the single factor responsible for more fouled up varnish jobs than anything else.

Choose the right weather: varnish indoors if possible. Temps between 50°-80°F are the entire range where you can expect good results (ideal: 55°-65°F, with 45-55 percent relative humidity). Don’t varnish in the wind. Dust is the enemy of a smooth, flawless finish so you have to minimize its impact whenever possible. Don’t varnish in direct sunlight because uneven drying can cause wrinkles. Avoid varnishing in high humidity or when rain is in the forecast. If you work in open air, time a fresh coat so it can dry before nighttime dewfall sets in."

This seems to be fairly common advice on the Internet.  I see several folks saying that if they varnish in high heat and humidity, they thin their varnish...but very sparingly (about 5%) and using the varnish manufacturer's specific thinner, although my friend recommended odorless charcoal lighter fluid as a varnish thinner. I live in Southern California, and he said because of air pollution regulations, paint thinner sold here is terrible. He said my Lo VOC Petit Flagship is half the strength folks in the rest of the country use. Go figure.

 I suspect the problem you'll have varnishing outdoors is not with the humidity, since you'll be moving the boat from high humidty into lower humidity. The problem you'll have is with varnishing in the sun, so that the varnish's drying agents will evaporate before the varnish has had a chance to smooth out, and because of wind blown dust. Better, I suspect, to borrow some shop lights to improve the lighting in your garage,and perhaps varnish in the cool of the early morning.

I have to tell you that I've read a lot of frustrated reports on varnish problems, and I get to the point where I think you need to make sure you start varnishing when the full moon falls on a Thursday on the day after the Chicago Cubs win on the road.

Good luck.

...or you could skip all the hassle, and get a harder, shinier, longer-lasting, and much easier to apply finish with marine urethane.  -Wes

Wes, do you have a reccomended clear coat urethane?  I'm a novice having only built one vessel.  The varnish stage was the least satisfying.

  Hi Rich,

  I may have some helpful info, I live in FL and have learned from a few of my own mistakes. No air conditioner but I do sometimes use a dehumidifier that does a good job but the drier air onleashes dust and makes keeping a wet edge a footrace around the boat. 

  The first thing I recommend is getting a hygrometer so you know what the humidity in the shop is. I have one that works well and was under $10 - tbe brand is Lockdown and it's intended use is for gun cabinets. 

  Second is a thermometer. Then find an app for calculating the dewpoint, plenty out there but this one works for me: link 

  Once I determine the dewpoint I stay 7° away from it when applying epoxy or varnish. Often the humidity in the shop is much lower than the relative humidity and if you can work around the dewpoint without drying the air it may be easier for you and less dust on the shine.

  Unrelated but maybe helpful - I've given up brushing varnish and now wipe it on, you'll never see me looking back! I thin the varnish about 30% with mineral spirits ( I've read you can thin to 50%) and use a stain application pad which is just a sponge inside a cloth bag to apply. No need to tip and you can reapply in a couple of hours. Downside is the coverage is less and you need more coats but you can put five on in a day. There is a learning curve and you still need to watch for sags/runs. 

  How about getting a headlamp, leaving the boat in the garage and varnishing there? When I closed in my carport for a shop I went crazy with flouresecent lighting, there's six 4 tube fixtures on my ceiling and I still use a headlamp to find the sags!

 I would worry about 'frosting' if the varnish was applied outside and then moved into the air conditioning.

  HTH

I think the WM info is for a boat outside rather than in a shop. I would do it inside with the a/c off. You don't need more air movement. For lighting I would buy two sets of work lights on stands. They are relatively inexpensive. If you have flourescent light leave them on after you varnish. I read they drop their static charge and dump dust when turned off. I see a difference. I would varnish even if there is high humidity. Keep in mind your last coat very well be the next to last coat. 

I've had excellent results with Minwax Helmsman Spar Urethane. I wash my boat after each outing, and store it indoors between uses, and it still looks like new after four years of hard use. For the ultimate finish, if cost is no object, have an auto body shop apply two-part urethane clear coat. -Wes

I am amazed by the responses to my vqrnish question!  The time and effort expended, knowledge, experience, and intelligence reflected by them blew me away.

Once again,CLC forum contributors rule.  Thanks.

RichMPL