I am also interested to hear what others do because after 8 kayaks and a skiff I am still not sure what the best technique is.

I beleive that the purpose of the DfD is just to seal the wood and prevent water intrusion. 

On my first couple of builds, I significantly oversized the holes but then had trouble centering the second drill as you suggest.  Since then, I have drilled the holes 2-3 64's oversized, then aggresively coated the inside of the hole with epoxy (watch out for drips on the inside).  Once the epoxy cures, the hole is still there with a diameter that usually allows the fastener through with out drilling again.  I do all the drilling prior to varnish so I also make sure that I get some varnish in the hole for good measure.

So far I do not have any water damage around any of my holes.

I try for a 1/16th or 1/8th annulus of epoxy around the fastener. Usually my process is to accurately mark the location, then use a pointed tip predrill/countersink to center the bit exactly on my mark, then cleanly enlarged to a 3/8" perfectly centered hole.

Add tape to the backside, mix a no sag mixture of epoxy and your filler of choice, then tool it into the hole, finishing with a pass from a round-tipped tongue depressor tip, leaving a smoothly concave surface in the hole. This divot shape can be done early in the build, and various sanding, coating, painting or varnishing won't hide it.

When cured, a fastner-sized drill bit will naturally center itself in this divot. The slight recess around the fastner from the remaining part of the divot provides a reservoir for whatever sealant your hardware is bedded on.

Then the question of how to do the DfD depending on whether it's a through-fastener vs. a screw that only goes in to its length.

What's been suggested above ought to work well for through-bolted fasteners, essentially 'pillar-bedding' them so there's no wood fiber being compressed; the fasteners are snugged down onto a bushing of filled epoxy some dimension larger than what would be minimum to allow the fasterner to pass through, then drilled for that clearance bore before fastener is inserted.

Wood / sheet-metal screws require a different approach? Will tapered threads hold well in solid epoxy? Does threading into epoxy require a longer or larger fastener - or both? - over what would otherwise be adequate when set into solid wood?

Where possible I'd go with through-fastened hardware for security at the expense of weight but there are places where a screw might be enough, or possibly the only kind of fastener that's even possible.

I really miss that edit post button.... 

  I’m really lazy. My go to technique is to drill the hole, thoroughly coat inside with unthickened epoxy, coat the fastener with mold release then slather it and the hole with thickened epoxy and screw/slip it in the hole. Occasionally a bit of tape is required to keep it centered. When the epoxy is mostly cured, say, just to the point a thumbnail won’t dent it,  I back the fastener out for the last bit of cure. I’ve gone as far as making tabs from HDPE to fill a specific slot. Drilling the second time is a drag. There are lots of variations depending on specific application.

About maximizing wood screw bonds, and waterproofing:

First, all this stuff is covered by the Gougeon book now available as a free download at the West System site. Most of us no longer use their goo, but they did much of the original research into its effective use in a marine environment in wooden boats.

Basicslly, epoxy has a much higher crush strength than wood, especially the soft woods we use for things like shear clamps in lightweight boats. Embedding a wood screw into an oversized epoxy filled hole makes the screw have something tough to bite into, and creates a larger surface area of wood now being used to hold the screw's pressure. The trick is installing a screw into an oversized goo filled hole...

Careful drilling is the answer. Predrill for the screw's shaft, then carefull chase that hole with a larger bit that is less deep by maybe a 1/4" than your fastner. The screw tip will bite into the wood, the bulk of the screw will be potted with an annulus of thickened epoxy. The thickened epoxy is injected/tooled into the hole, the hardware placed over the hole, and the screw inserted through the goo and gently tightened in the tip hole.

I've used this for attaching back bands to my cedar shear clamp, where 1 screw in  really soft wood is carrying a pretty stout load. It's waterproof, and doubles or triples (whatever the Giugeon book says, I forgot) the screw's load capacity.

Interestingly, they found fully threaded machine screws worked better than wood screws in high load cases. The threads better engage the epoxy, and the head to shaft transition is often better designed than sheet metal type screws. The predrill for the tip is just a snug fit for the screw's minor diameter, and installs the same. 

Good stuff here! Thanks!

   So I've been seeing this DFD and my main question in plain English is what is the objective of it and in what scenarios is it useful.

I'll be finishing up my MC 13 w/ CLC sailrig over the next month. All parts built so basically finishing and putting it all together, rigging etc. Just curious where it might be a benefit for me to learn and use this technique in this part of the build.

Newbie on my first build so pardon my ignorance and if I might have missed this explanation somewhere. I'll be checking the aforementioned Gudgeon book if that relates. Not being lazy just good to hear different points of view. Thanks  for your help. PP

Here's a link to an essay on fastener installation in wood by a renown naval architect, one of whose designs is featured among CLC's array of kits, his W17 trimaran.

He goes into much greater detail on the whys and hows of this process that I could ever manage, goes on to provide some interesting reading too about how he's done testing to evaluate the benefits of further techniques to improve the effectiveness of fastener installation.

The primary objective of DfD is to seal wood grain that's been exposed by drilling through the epoxy+fiberglass matrix applied to wood surfaces. Further addition of sealants when mounting fasteners serves both to augment this protection as well as provide a measure of insurance that fasteners won't loosen during extended use.

Mike's website's a great resource for those interested in learning more about what he's found useful about designing, building and maintaining boats of many kinds. Had I been aware of it when I originated this thread six months back I likely wouldn't have bothered to ask the question.

    @spclark. Thanks lot of info there I bookmarked and will refer to as I go.

Goal is to build it once and not have to fix it forever but again it's a boat so that's probably not going to happen. PP