Link to photos here since the last one is not editable.  https://imgur.com/a/we4jX93

"...I wonder if this is what varnish that is going bad looks like."  

No, not really. In fact, loooking at that image you've linked to your varnish job still looks pretty darn good after two years, at least to my eyes.

Degraded varnish looks cracked in most cases, really bad stuff turns to powder almost with every touch. You don't want to wait until you see that happening; common practice is to sand lightly then add a fresh coat or two periodically.

Images included in this series on the Waterlust folks' Teardrop build ought to give you some reference for evaluating your Teardrop's appearance after two years.

Keep in mind that varnish is what protects your epoxy-clad marine ply from eventual degradation and subsequent damage from the Sun's relentless UV radiation. Even in the shade there's usually enough UV component to cause most varnishs to begin to yellow. This is what they do, the yellow filters out the UV just like yellow sunglasses.

Did you know there's a separate forum for Teardrop Camper builders? You may want to look into that for a more thorough discussion of the phenomena you're frustrated by: http://teardropforum.com should take you there but right now I'm getting a database error... no idea why.

"Are you supposed to sand all of the varnish off?"

Unless you used a catalyzed varnish you can use conventional pain strippers to remove old varnish. Even then, once most of the varnish has been stripped away you'll still need to sand the underlying epoxy before starting a fresh coat.

The catalyzed stuff, being tougher than its one-part cousin, needs physical removal, usually by sanding. For a project as large as a Teardrop I suspect sanding is the most economical choice.

Hope this helps.... 

Varnish looks fine. I'd say that either it was the different pieces of wood reacting to the sun differently. That'd be the result of coming from different trees and/or being processed differently at the plywood factory.

When I re-varnish I wet sand with #400 paper. That lets me remove the varnish without any dust and without hurting the epoxy surface. While I'm wet sanding varnish it deels sort of soapy. When that feeling goes away I know that I'm down to the epoxy.

Laszlo

   Thank you both for the replies.  I guess my varnish is in decent shape but the yellowing is "natural" for it not being completely covered.  I reckon I will plan on sanding lightly and then adding a fresh coat (we used whatever CLC recommends, will double check that there is not something a little tougher) in a year or so, and then keeping it under a cover.  

Also good info for our NE Dory, which we plan to paint the hull and varnish the interior.  Definitely going to invest in a cover straight away.  

   I'll ask how many coats of varnish you initially put on the camper.  With an amber varnish more coats does lead to a more "yellow" color, even on a new finish.  I ask because I think more coats are better during initial finishing (at least 4, but with 8 or more getting into the more than needed territory).  First, that is the easiest time to varnish - fresh clean surface with limited fittings being in the way, and motivation usually at its highest.  And the reason to do so many coats isn't because the finish looks any better after the 3rd coat.  The reason is because U/V, contaminants like chemicals and salt, and oxygen are what degrade varnish over time.  It becomes brittle and the plasticity is gone, then thermal changes lead to micro-cracks, at which point the microscopic surface area rapidly increases, and thus accelerating the rate of further degradation deeper into the varnish surface.  And the first time a crack allows moisture to reach the wood, degradation rapidly accelerates, including debonding from the wood and oxidized (gray areas) of wood.  So if you put on 3 or less, even though your surface looks good, I'd consider doing a few extra coats now.

I used to live aboard an old CT 41 Ketch (lots of wood trim) in San Diego and thus had plenty of time to hang around the dock, and being young and single, had good reason to take note of my "surroundings." So, I base my recommendations on experience that I've gained after following the advice I got years ago from some young ladies that made a living doing little else besides hanging at the marina doing varnish day after day ("Bristol Fashion" (as they say) when they finished a job). I think the big trick to maintaining varnish is to put yourself on a schedule (very environment, storage and usage dependent, and dependent upon whether your varnish is over epoxy or bare wood) where you lightly wet sand (just as described by Laszlo) every 2-6 years.  You'll only be removing the first couple of coats of varnish, and the underlying varnish will have been protected by those top coats.  Then add back 2-3 coats of varnish.  Although this sounds like a lot of effort, it is MUCH easier than waiting too long and having to recover a "failed" varnished surface by stripping down to bare wood (or bare epoxy).

Added benefit of the multiple coats should be many many years before you start seeing your epoxy cloud up.  Sooner or later that will probably happen, but by then your boat will be so well-loved you'll consider that a proud patina of age (or at least you can pretent that you do).  

I don't really care if my boats look glossy or has some minor varnish drips and blemishes.  So, it isn't about the shine, but for all the reasons above concerning protecting varnish from becoming yellow/brittle with age I wax my boats almost monthly while in season.  I know others don't, but that wax takes minutes to apply, and might add a couple of YEARS between varnish jobs IMHO.  Good high carnuba content wax with U/V protection.  I've tried many; this is my current favorite: With all other things being equal among some good waxes, being able to quickly buff out a wax makes the job much easier. Meguiar's G7014J Gold Class Carnauba Plus Premium Paste Wax, Creates a Deep Dazzling Shine - 11 Oz Container

-Bubblehead

  Four coats.  

Looks like we will be doing light sanding and adding two more coats or so this coming winter, as well as getting a cover for it.