The hull is West System two part epoxy and the deck is spar varnish.
Can I apply a fresh coat of varnish over the existing coat without sanding off all of the
existing varnish?
Oh, hey thanks for replying!
Well, what I would say is this...
You have two main options as I see it.
Option 1 (The nothing to lose approach) Give the existing varnish a cleaning with something like TSP and then lightly sand it to remove imperfections and cut down on the sheen of the existing spar. Then you'll obviously have to tack cloth it to remove all the dust. After that process, you would then apply another coat or two of fresh spar varnish.
The risk in this approach is usually related to getting good adhesion between the old and new coats of spar. Perhaps you could check the instructions with the spar varnish manufacturer to see what they say? Typically what they'll warn against, is the fact they can't guarantee their product will stick to existing finishes.
Also, you mentioned that some of the existing varnish is flaking. To be honest, that might be the true fly in the ointment with the scuff sand and recoat approach. The reason being, that applying new varnish might start to lift more of the exising varnish if it's un-sound and flaking. So you'll want to remove all or at least as much of the un-sound existing varnish as you can. And because you don't want to, or can't restain, you'll want to avoid sanding right through the varnish and into the stained wood as that will obviously cause color consistency problems.
I guess the bottom line with this approach is that, at the end of the day, what do you have to lose? If the varnish doesn't stick or hold up properly the only other option is option 2.
Option 2- (The by the book approach) Sand off the existing varnish getting back to bare wood, restain and revarnish. If you go this route, what I would do is make sure to get a good cover to protect the boat as much as possible. And then you'll probably want to put a coat of fresh spar varnish on every few years to keep things fresh. And you'll want to do the recoats in advance of the spar starting to fail.
Just my .02 cents.
Let us know how you ultimately make out.
-Chris