Epoxy hardens through chemical reaction, generating lots of heat. You should be OK. Best if it's not actually raining and the dew has dried. You don't want any direct water contamination.   

Varnishes and paints dry when the diluent has evaporated and only the binder and tint is left. Evaporation rate is affected by relative humidity (RH).

Epoxy cures via endothermic reaction. Some epoxies, like some cements, can cure under water.

I've not seen RH mentioned in epoxy datasheets. I've seen it mentioned in marketing for MAS and WEST Systems' 207 hardener where the latter states "it will not blush or turn cloudy in humid conditions".

   Pedant alert:  That's exothermic for sure, as in generate heat while curing.. :-)

So long as it's not condensing on the surface before you start, or the surface has free water on it, you should be good.  Protect it from rain w/ a tarp until cured so you don't get spots.  Varnish, as you know, is a whole 'nuther matter.

Thanks Mummichog, exo. My Ancient Greek is a little rusty.

PS: that unnamed poster was me, apparently what happens when a draft post sits in the browser all evening and the session times out. 

Thanks guys! It makes sense. I mixed up some thickened epoxy yesterday & everything went well. Moving forward for the next few days when temps will be reasonable (not so hot!) to work in the shop even though humidity will be a little higher.

Can't wait to move on to the finishing phase of my boat! I'm glad I got a digital temp/humidity meter for the shop. Didn't know it would be as important to the project as it has become.

Take care,

Frank