Use the best sandpaper you can find. What I've found works best is a 3M product that simply outlasts pretty much every other type I've used the last 40 years.

Make sure your epoxy's fully cured (depends on ambient temperature during curing as well as the epoxy brand and choice of hardener used) otherwise it will clog your paper prematurely.

If you have really rough areas to deal with, using a cabinet scraper or similar will vastly speed getting things flat. Save sanding for the final pre-finishing prep steps.

For details like sanding rounded edges, 'break' the sandpaper over a durable edge first along both its length and width. This reduces its stiffness, allows it to conform to non-straight edges much better, reducing sand-through a lot. You can fold it into a supple pad or wrap it around your choice of suitable sanding block for really careful use.

Try not to overuse a given piece of sandpaper.

If using a power sander, keep the speed low. Fast speeds can generate heat and can soften epoxy, lead to faster clogging and may cause drag lines from the accumulated gunk.

Wear your respirator; epoxy dust isn't something you want to be breathing.

In addition to the splendid advice above, you might find a set of these:

https://www.clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/riffler-rasp-set.html

...useful for working out bumps and lumps in fillets which would otherwise wear out your sandpaper and patience.  One of the various shapes will be just the thing for carefully levelling a spot out so you can sand it smooth, with much less likelihood of digging into adjacent places you don't want to dig vs. some sort of power tool, e.g., Dremel tool, grinder bit, etc.

.....Michael

I just did the math, and I am about 130 hours in on what is supposed to be a 200 hour build.   I still have to cut and shape mast and spars, and do things like attach rudder and centerboard and misc hardware (they are already shaped and epoxied).  lets be generous and say 20 hours for that.  that means 50 hours sanding, epoxying where I oversanded, painting, sanding,varnishing, sanding, painting & varnishing again and sanding etc.  I feel like I might have entered a ring of hell. 

At least I have plenty of n95 type masks that I have just lying around, on account of the global pandemic or something.  Silver linings I guess.

Just a couple of points that are not directly related to sanding technique but have a major impact on finishing your boat. First, ignore the 200 hour estimate, especially if this is your first boat. While it's possible for someone to actually build a boat in the number of hours stated by a manufacturer, it's not necessarily typical. There's too much variation in circumstances, such as skill level, available time, available equipment, life situations, etc. That's the main point behind boat building classes. They provide focused time and effort, consistent tools and materials and an instructor who is a skill equalizer. For the rest of us, a boat is finished when it's finished, regardless of what clocks and calendars say.

Second, don't worry about completing the sanding. Focus on completing a particular seam or fillet. When that's done, look at it, appreciate and admire what you just did, pick another and do it again, knowing that you've done it before and can easily do it again. Before you know it, you'll have run out of stuff to sand.

It truly gets easier and faster as you go. Effective sanding is a skill, just like any other. And as for the eternal cycles of oversanding and applying new lumpy epoxy, they can be broken by using a chip brush dipped in unthickened epoxy to smooth the still soft fillet to a glass smooth surface that needs no sanding.

Good luck,

Laszlo

Nate:

That's the second time you've used "hell" to describe your state of mind regarding the finishing of your Peapod.  I, too, find that my "finish work skillset leaves a little to be desired", so I know something of what you're feeling.  Well, okay, truth: my "finish work skillset" is practically nonexistent.  More of a boatwrong than a boatwright, I am.

Perhaps you should think of working on your skill with some small, bright finished project--birdhouse, spice rack, doesn't matter--to see if you've got the ability to improve your skill to a level which would give you some confidence when it comes time to apply that to your Peapod.  Try not to set yourself up for a self fulfilling prophecy.

On the other hand, if the prospect of varnishing the interior of the boat is keeping you awake at night, maybe it's time to rethink that whole varnish business.  I mean, for folks who have the knack, dealing with acres of complex varnished surface is fun.  For the rest of us, it starts to sound like Sisyphus pushing his boulder up the hill again and again.

If you're really committed to the idea of a bright interior for your peapod, and you feel like you can pull that off to your own satisfaction without spoiling the fun of the project, take your time and go for it.  Or, perhaps you know someone who has the knack and is willing to help you along.  Projects like this are a lot more fun if you have family and friends share it with you.  I know our Passagemaker Dinghy build ended up being a three generation family project, the memories of which might be more important than the boat.

In the end, I don't know that anybody is going to turn up their nose at your Peapod if all of her slight imperfections (inevitable) are lurking under paint instead of highlighted by varnish.  The lovely shape, the exquisite lines of the plank laps, and the way she moves on the water will make even the fishermen in metal flake bass boats turn, look, and smile.  In the end, she's a boat, not a museum piece, and maybe worrying about the "museum grade finish" is best left to the curators at the museum to which you donate her when you've grown too old a feeble to go boating any more.  <;-)

We decided to limit the brightwork on our Passagemaker Dinghy to the bow and stern transoms and the corresponding doublers on the inside.  The exterior was done with glossy white Behr porch paint, the interior with a semigloss warm tan they called "cold lager" at the time.  Easy to work with, easy to touch up, and it's held up pretty well.  She turns heads wherever she goes:

https://www.clcboats.com/file_uploads/photo_contest/65339834429.jpg

...if that link remains stable.  Another photo of her once won a monthly contest, others have placed 2nd or 3rd, and several are in the CLC photo galleries for the design, mostly the take apart version, which she is.  (By the way, I won't object if you want to go to https://www.clcboats.com/voting and give her your vote, if you think the photo worthy.)

In short, calm down, take Laszlo's excellent advice, and don't let the devil rob you of your joy.  If you start to feel stressed, make sure somebody has your Sawzall locked away where you can't get at it.  <;-)

.....Michael

   Here's a vote for painting the inside and outside plywood and save the varnish for the rails and other detail items.  Partly because they'll be the solid wood parts that are less likely to have visible lumps and bumps that challenge the sander.  And, painted surfaces can be faired w/ microballoon mix if the divots are too big to easily sand.  Also, I used the Interlux Flattening Agent in my Brightsides paint for my exterior and the less glossy finish is good at hiding minor imperfections.  Lastly, evaluate your finish from 10 feet away, not 1 foot.  I have lots of finish issues on my Skerry and I still got the attention of a professional on-water photographer while I was sailing through the Annapolis mooring field, in the middle of a bunch of shiny yachts.  

   All excellent tips.  Mummichog, if it proves too difficult, I will absouletly paint inside.  I'm going to try bright finish first and see how it goes first.   I'm only going to get one chance on it, so I might as well try. 
Good news is my standards for finish are solidly not very high.  a few wavy lumps are going to be plenty fine with me.   Just not cringy lumps everywhere.  I have put about 6-7 hours sanding the outer hull and seats, and while no where near done, I am making incremental progress.  And whil I know I won't be magically done when I hit 200 hours, I am keeping track of my hours to see what my time vs estimated time ends up being, perhaps in case I ever build another, or someone asks me how much time I spend building it.  if it ends up being 50 hours of sanding and painting, so be it.  I am in 7 hours already and I could easily see spending 30+ plus hours sanding alone based on my progress.  I'll just keep plugging away at it maybe couple hours a day, and who knows, maybe I can get it done in a month or so. 

 You're worrying too much the finishing work is alot more forgiving than you think .go with the schooner varnish and not the satin, I got that once by accident and it looked really bad. Use stop loss bags for your varnish