Wait at least 24 hours before varnishing, but the longer the better.  72 hours is definitely enough.

FrankP 

I just finished a build.

Overnight between wetsanding with 400 and another coat is fine.

3 coats and I called the boat done. Looks great. Didn't see the need to go the 5 and 6.

It got wet for the first time yesterday!

Congrats Fishbuster.  I've decided to do 2 coats a day of varnish on the hull.  Then do the deck 2 days from now.  That will be 3 days of cure time for the deck epoxy. 

Thanks All

Hate to rain on your parade but it may take a bit longer.  Some varnish does not cure very well over fresh epoxy.  For example I have found it best to wait at least 2 weeks when using Interlux Schooner 96 varnish over System 3 epoxy.  Do a test patch before you varnish the whole yak to be sure you are good to go.  

Don't know about system 3 Steve, I used MAS epoxy and slow hardener.

If the epoxy is cured enough to sand, seems to make sense it is cured enough to varnish. I used the same Schooner Varnish. No problems to report.

You're both right. It pretty much depends on the temperature. The epoxy has to be completely cured so that it it not giving off any chemicals which could contaminate the varnish. In a Mid-Atlantic states August, that could actually be less than 72 hours. In the Pacific Northwest that could be a month.

Waiting until it's sandable and making sure that the surface is thoroughly clean is a good way to go.

Laszlo

Laszlo makes a good point about the weather.  The brand of goo and hardener selection makes a difference as well as the brand of varnish.  My comment was specific to my experience with System 3 and Interlux Schooner varnish to make the point that you need to be aware of what you are using and not make blanket statements about how fast you can varnish.  Another thing to consider is that "cured hard enough to sand" is not fully cured for most resins and glues or for that matter paint and varnish.  Full cure can take far longer.  I usually figure epoxy is cured fully after a week at normall room temps.  Good to hear that MAS goo is ok to varnish so quickly.  I have also heard the Epiphanes varnish is good for short cure time epoxy as well.   

No, ya know, it is a loaded question now that I read it again. I must have missed something reading it before my first shot of coffee. I am talking time between varnish and weather does make for huge variations. I did not varnish my boat overnight after the last coat of epoxy. Overnight was for all the varnish coats, once varnishing began. I think it impossible to varnish 12 hours (or even overnight) after the last coat of epoxy. It needs to be sanded first, after the epoxy dries, this is all too much to happen in 12 hours or so, in any weather.

By the time the epoxy dries, then the boat is sanded, then the shop is cleaned up thoroughly, the first coat of varnish, being practical, is a couple/few days after the last of the epoxy was wetted out.

Slow Down! enjoy your work Most epoxy Manufacturors will tell you that depending on shop conditions a safe minimum would be around 72+++ hours this is a chemical reaction and by sealing in varnish so quickly you will affect the end strength of the cured epoxy  you are building a craft that CAN last 50++ years it may not seem relavent today  but if you insist on rushing yopu may cut your boats life short  I would expect you to go paddle your boat and then get down to the finishing Always remember PATIENCE

and oh yeah mister three coats better think twice about UV protection IT IS NOT ENOUGH, maybe for a few paddles, but with use you should consider 6 as a minimum besides with six your boat will look like a fine boat and three just will not want to set his besides yours 

C Fox Boat School

www.cfoxwoodboats.com 

Yabut Okoumefest is in 9 days!!!

Quality epoxies don't contain solvents and do not blush, so will not be affected by being sealed away from air after their initial cure (undentable by fingernails). Another reason to pay a bit more for the good stuff.

3 coats can be enough, depending on how often and where the boat is used, where it's stored between uses and how often it gets revarnished. But here in MD I'll use more coats.

Gotta go sand and put another layer of epoxy on my Duck 12 now, bye all.

Laszlo