Why mask the gluing surfaces?

Most builders I've studied seem to mask the gluing surfaces while sheating/impregnating. I've tried to find a reference in François' instructions and Ian Oughtred's book, but I haven't found any.

AFAIK, epoxy is supposed to bind very well on both well cured and 90% cured surfaces, as long as they're amine free and/or lightly sanded.

Then, why would you want to mask the gluing surfaces? Isn't the risk of skipping an area (for example in the buoyancy compartments) greater than the possible advantage of slightly better binding?

Hi, Prompted by PaulW I'll do my best to reply to this.
Caveat, I'm not a professional, this is just how I do things, and it's worked fine for me so far, five boats in.

To begin with I've read quite a few books and everything written in Watercraft over the last 15 years or so and I'm not aware of people masking the gluing surfaces, which might explain my reluctance to answer the question. Maybe I'm misunderstanding.

The question also implies that the sheathing and impregnation is happening before the parts are glued which is not the way I'd normally do things, however I do recall sheathing the bottom boards on the Morbic before assembly.

I don't see any need to mask off the gluing surfaces, on the bottom boards any surplus glue will be planed off when the edges are bevelled.  

My process for planking would be to plane/cut the bevels lands etc until I'm happy with the dry fit.  
Mark a point mid way along both planks to help line things up when you re-fit.
Mark the lap in pencil on the new plank.
Optional, mask the outer edges of the glued surfaces to help remove dribbles.
Prime both surfaces with neat un-thickened epoxy.
Immediately run a bead of thickened epoxy along the land and offer up the  new plank.
Gently clamp from the middle outwards towards the ends.
Once happy with the fit - don't over clamp - clean up any surplus glue and fill in any gaps.
After the clamps have been removed, maybe a couple of days, I always run a fillet of thickened epoxy into the exposed edges of each plank edge to seal it.

When filleting the inside of the boat I usually mask off the edges of the fillet to give a neat line and make clean up easier. Remove the tape immediately, don't let the epoxy go off or you will struggle later.

Use any surplus epoxy left in the pot to fill screw holes, gaps etc.
A hot air gun and scraper are your friends when cleaning up.

If you are unfamiliar with epoxy, I suggest you assemble/sheath small parts; rudder centreboard etc. first.  

Hope this helps, any further questions just ask.
Others might have other ideas, feel free to pile in.

Merry Christmas all.
 

Thank you very much, it's this kind of information I was looking for. I didn't mask any gluing surfaces on the strakes either, but it seems somes builders do it preparing the centreboard case. Anyway, from your info and some ideas from Paul, it seems I don't have to worry too much about masking.

i recall being surprised, before we started building Proteus, when I saw a friend's freshly started build.  "Why", I asked, "have you masked those areas?"  He (building a Goat) explained they were areas where structural bits would subsequently be bonded.  If i remember correctly, the received knowledge (or technical instruction) was that it would be a stronger bond if it was to FRESHLY epoxy-primed virgin wood than to a previously-epoxied and then sanded surface.

This seems to be backed up by this information from "West User Manual".  This may be the new link, then go to Page 29 "Primary and Secondary Bonding"
In particular see pages 28 onward, specifically the section explaining "Primary and Secondary Bonding".  Doesn't it give the impression that Secondary Bonding (mechanical) is less desirable than Primary (chemical).  One would imagine the people at West know a thing or two about bonding!

Now see this item on "Boat design Threads" forum.  Scroll down to Glen Tranchon Dec 2018.  "The bonding strength of most epoxies is around 2000 psi. Polyester is usually around 500 psi. A secondary epoxy bond is also just as strong as a primary bond with polyester.".

Or you might find this more convincing. "Five reasons why two-step bonding with marine epoxy creates a better joint"

I don't know who or what to believe - Graham has worked with epoxy much more than I, but if I was building a boat again (unlikely) I'd probably take the trouble to do the masking (again) so that structural bonds could have the benefit of the doubt, ie primary rather than secondary bonding.