I am working on an Annapolis Wherry and have...  also applied one coat of epoxy to all other non-glassed surfaces.

I ... have a couple questions regarding the unglassed areas of the boat:

1) On the non-glassed areas, Is there an easy way to tell if you have sanded through the epoxy and have exposed the bare wood again?  It seems hard to distinquish if everything is kind of gray-white.

If grey-white's uniform then you've not sanded through the epoxy. Once sanded through you'll see what you'd seen before you started in with epoxy encapsulation particularly once you start vacuuming off the dust you've raised.

2) I have heard different opinions on the application of additional coats of epoxy on the non-glassed areas after the first coat...

I've only bult one (CLC kit) but the general recommendation from CLC as well as other sources says two coats – minimum – is what will best keep water away from wood. Three is better yet unless you plan to paint or varnish what's exposed, whether inboard or outboard.

3) If I am sanding in preparation for another full coat of epoxy, do I need to sand to the point of getting a "uniform white" surface, or would it be OK to just sand to what I feel is adequate, and then use a scotch-brite pad to rough-up the areas which did not get touched by the sanding pad?

Do your level best to sand everywhere you possibly can before adding another epoxy layer.

Unless the underlying layer is still 'green' (i.e. not yet fully cured, which takes both time and proper temperature to achieve) bonding of subsequent coats will always be suspect.

Chemical bonding happens when the underlying surface hasn't cured 100% yet. Sanding uncured epoxy is best avoided as it's both difficult (epoxy will gum up sandpaper) as well as unhealthy (uncured components are toxic and should be avoided, particularly hardeners).

Know also that heat from sanding cured epoxy will cause the stuff to soften some and will shorten the life of your sanding media. It's common to undo mistakes with epoxy using a heat gun to soften it for removal.

Learn to follow manufacturers' recommendations about cure conditions for the products you're using to know best how to protect yourself from harm and get the best performance out of the product your're using.

Principal behind sanding cured epoxy serves to achieve two things at the same time: 1) you're taking off some of the last layer so there's leveling going on, taking off high spots; 2) you're adding texture, however minute in scale, giving an additional layer something mechanical to bond to.

(My take on this has the mechanical aspect of sanding/filing epoxy breaking what had been complete epoxy molecules on the surface that then are available to bond to fresh epoxy once that's applied. I'm no epoxy chemist though so my theory remains just that.)

I'm a fan of Scotchbrite and similar non-woven abrasives owing to their ability to conform to surfaces that otherwise would be difficult to sand well. Among other builders I've communicated with this is a shared opinion.

Just be sure to remove as much of the resulting dust from roughing up epoxy surfaces as you can, first by vacuum then a damp wipe down with clean water, before you add more or start further finishing with other materials.

That wipe-down is when you'll see a difference between raw, exposed wood and adjacent areas that have been covered with epoxy.

Using solvents like alcohol may introduce substances that inhibit further bonding unless you trust their source 100%. Even then solvents cost money and have issues regarding safe use and disposal while water's both effective and inexpensive.

If you're using the MAS LV epoxy system sold by CLC and others, what epoxy blush issues might otherwise arise will be avoided. Blush is something that occurs when epoxy hardener reacts with ambient humidity to form an amine coating at the epoxy / air boundary. Its presence will seriously compromise subsequent epoxy, paint or varnish appplications. Fortunately blush is easily removed simply by water washing and Scotchbrite is an effective aid in this as well, ensuring its 100% removal.

Hope this helps you in your build!

   Thanks for that detailed information.  It makes sense, and I have likely read about it previously, but this definitely helps my understanding. 

It has been three weeks since I rolled on the initial coat of epoxy on the non-glassed areas, and I do believe it is cured at this point.  I will begin my sanding journey today! 

Scott

Periodically vacuuming off dust then a water-damp wipe with clean paper towels as you progress will reveal any areas that have been sanded through if you're not sure of what those areas may look like. Plywood, dampened with water, will appear darker than the surrounding areas.

Just be sure you've allowed ample time for any residual water to dry out before fresh epoxy's laid on.   

With a knife tip, you can gently scratch the surface to see if it has epoxy on it.

When the wood shows, the glue is gone.