Answering a question with  question: If the prepared surface is alreay smooth and flat, what's the benefit of taking it that far?  You're going to clear coat it with varnish anyway, probaly several coats, with sanding in between.  Unless you just love that sanding and, in that case, come on over to my shop, I have a job for you.

320 wet or dry is the perfect end to your epoxy sanding congradulations on being willing to go the distance and get the base ready properly the more you perfect the substrata of course the better your final finish can be, As we see students working on finishing the difference between those who persist and those who just have to go paddle it now is incredible for those who are willing to take the time to get it right the rewards for the hard work of sanding will come when you pull up next to a hurry up guy(or Gal) and see the crestfallen look they tend to have on there face knowing full well that you took the time and they did not ENJOY the process this is by far the biggest impact step we find the step from 180grit on a RO sander to 320 wet is almost perfect

all the talk I see here sounds as if most builders are thinking a couple of coats of Varnish is enough, well I would suggest that  you  do a little research on the need to protect the e-poxy from UV degradation you have probably put a lot of time and thought into your pride and joy and if properly finished and cared for it can last your lifetime you should protect that E-Poxy with a minimum of 5 coats of high quality marine spar varnish and if youwere to speak to the e-poxy experts they would tell you it should be 6-10 and to renew annually with a lite sanding and 1-2 more coats

cfoxwoodboats.com 

"Answering a question with  question: If the prepared surface is alreay smooth and flat, what's the benefit of taking it that far? " 

 Jack, your post sets out my issue exactly.  As I stated above, I have years of experience with metal refinishing autos, trailers etc.  In those projects you have to get the foundation coats smooth because any surface flaws will show up some way in the finsh coat. 

This Is my first boat project and I have several concerns.  in auto finish you have both a mechanical bond and a chemical bond between film layers and you can get too smooth thus defeating the mechanical bond.  You would never sand a primer coat with 1000 grit or finer because you will likely end up with film separation.  

If I understand the boat process there is little if any chemical bond between the epoxy and the varnish.  Thus, I am relying on the mechanical bond between the films to maintain the finish and avoid peeling etc.  My inexperience prevents me from knowing what the best "scuff" for adheision would be in this context.  As I understand it we want to remove scratches that would appear through the finish but still leave things rough enough for a good finish.  I assume that just like automotive work, the more prep work done under the surface the less surface work required to give you a beautiful final finish. 

The CLC Shop Tips recommend 220 for the final epoxy sanding and 400 between the varnish layers. They also recommend a minimum of 3 coats of varnish for UV protection with more than 7 or 8 being overkill.

Once you get past the basic issue of UV protection you're into builders' intentions. These range from boats built strictly for utility (with rougher finishes) to boats built strictly for show (with museum piece finishes). Most are somewhere in the middle.

They nice thing is that since the finish is only the final .02 inches of the boat and has absolutely no effect on structural integrity or performance (at least at the speeds that a small human-powered boat moves), you can always go back and work some more on the finish later, if that's what you want to do.

The thing to remember is that no exquisite coffee table mirror-smooth finish survives first contact with the water. Carrying racks, trailers, boat ramps, sand, gravel, branches, paddles, water bottles, car keys, other boats, birds, fish, rocks, oyster shells, fishing lines, hooks, anchors, etc. - they're all out to get that museum-piece finish, and they will. They only way to preserve it is to keep it indoors and never use it. But in that case you've built a piece of furniture, not a boat.

Finally, finish isn't the only status symbol. Here's a typical utility watercraft with workboat finish only, but still expensive, exclusive and one you don't want to mess with.

Laszlo

Boy am I glad I don't have to paddle that!  Sail on boys.

paddle it...I'm glad i dont have to sand it!...my orbital would have a melt down. "Go Navy"

Me thinks it is interesting the "boat cranks" out there who have never taken the time, or maybe are afraid to take the time to finish it right, no you do not need to get a museum finish on your project !!!!! very few could acomplish the 15-20 coats of Flawless varnish it takes to win in that world, but again you do need to give serious thought to protecting your e-poxy and if you think 3 coats is what it takes you are dead wrong CLC states in virtually every manual I have (and believe me I have almost every one they have produced in the last 10yrs) that 3 coats is MINIMUM!!!!!!!! The leading authorities on e-poxy degradation due to UV exposure all state that 5+ is minimum, maybe you all want your boat to fade,yellow and start to break down after a couple of years and do not really care, but if you care enough to build it then why not care enough to protect it. as to the fact that it is only .020 of the boats structure if you do a little research you will probably figure out that it is without question the #1 most important .020 of your whole boat. Oh and by the as to the idea that if you take the time to do it beautifully that it will only get torn up in use well take a close look at the Mill Creek 13 hybrid on our website 3yrs old paddled 3days a week most of the summer flyfished out of regularly cartopped to 15-20 different shows and only one refresher coat of varnish since the begining and it was after these photo's were taken about last September 2007

Only with a complete finisn can you get longevity

cfoxwoodboats.com 

The Shop Tips also say 3 coats is a minimum and they are quoted as such above. 

This is a diverse hobby. People build for many reasons. Just as the type of boats selected for building are an individual choice based on each builder's situation, so is the level of finish. I'd love to do a civil reasoned discussion of the pros and cons of different finishes, but this thread has degenerated into namecalling.

Laszlo

Finishing takes many levels and forms if someone interpreted my coments as name calling I apologize that was not my intent I am just trying to get thru that this is an extremely important step that should not be hopped thru with the need for speed but done in a carefull workman like manner, varnish on the brightwork is all that stands between all that hard work and the damage the UV can do to your boat, some dont care about the quality appearence, most do once they have seen what they could have accomplished. In many shows we have done throughout the midwest the most common comment we hear is why does my boat look that good all we show is typically students boats and they usually look every bit as good as any "pro built" boat at the show what is an extra 3-5 days (total time maybe 2-3hrs on a typical kayak ) in the big picture with many of you having already put 100hrs or much more into your boat if you want it fast maybe paint or consider this excellent option 5 coats of Epifanes Wood Finsh Gloss (does not require sanding between coats up to 72hrs) can usually be recoated in 12 hrs depending on temp. and then wet sand with 320 and top with 1 or 2 coatsof Epifanes Varnish this may be the ultimate quick finish and it is extremely durable as long as you give it time to dry. Another note on that subject drying varnish takes time no product that we know of gets tough unless you are using a 2 part fro 5-7 days minimum I know I know you have got to go paddle but expect it to mark easily until it has truly cured

cfoxwoodboats.com 

guys please!  I wanted to build kayaks to relax from having to build those big grey things.....  and you have no idea the BS it takes to put a finish on those things.