Several hours later : I placed the hull out in the sun for a few hours to see if the warming would entice the 407 filler/epoxy mix to harden.  No success.

So I stripped the whole lot off and will re-mix and re-apply. (a) Should I clean the surface before re-applying, and (b) What do you recommend I clean it with ?

With thanks

Dave

I'm not sure what kind of gap you are filling, but I used West Six-10 Epoxy for the fillets on our MC 16.5.  I also used it to glue together some of the scarfs.  The stuff applies like caulk and the product literature says the following:  "perfect for filling voids and making fillets. It is ideal for stitch and glue boat construction, fiberglass laminate repair and general bondng."  I only used it in the main cockpit area and used wood flour thickened MAS in the fore and aft areas beyond the bulkheads.  The result was very nice fillets.  I taped all the seams before filleting, but if I use this stuff on another boat, I'll skip the tape.  Using the wood flour thickened epoxy was messy for me.  I may buy another tube to fill all the stich holes and the seams on the outer hull which will be painted.  The downside is the $20/tube price. 

Dave, to figure out what happened you might mix up another small batch as a test.  I had some epoxy that didn't properly harden, and think I spaced out and measured equal amounts of resin and hardener, instead of the specified 2:1 ratio.

Oops, I should have clarified that the 2:1 ratio I use is for MAS epoxies (and many others too).  But I see you're using WEST System hardener, which requires a 5:1 mix.  Obvious to you, I'm sure, but I don't want to create any unecessary confusion ...  

Jeff and OY,  Many thanks for your replies.

I finally worked it out.  West (and probably any other brand) epoxy is hygroscopic.  I had foolishly left some unused 105 and 206 in glass jars for about a week.  When it came to mixing them, I noticed that the fluid was a milky colour, but went ahead anyway.

On my second attempt the same thing happened, but I decided to discard the epoxy before adding 407. I then drew down on fresh material from the sealed containers. This mixed clear, as it should. Added the 407 and by this-morning everything had set perfectly.

All part of the learning curve.

That's interesting, Dave.  Haven't heard of that before.  I've recently been using some 10 y/o epoxy from containers that were stored the whole time with the pumps left fitted.  Not a single problem!

Here's an update O.Y.  The second batch of West 105/206 that I mixed, but did not use, was left in the mixing cup (I did not discard it ~ only the stuff in the two glass jars). The next day (i.e. yesterday) I thought I'd have a look at it and blow-me-down ! It had gelled.  Next question :  Why did it not gel with the 407 mixed into it ?

No longer a bother, but chemically interesting.

You mention having used 10 year-old epoxy stock (well sealed I presume). That's amazing.  John Harris mentioned to me that he had, on one occasion, used eight-year old West epoxy which performed perfectly.  So much for the "use-by" dates ~ which are probably set for extreme conditions.

Next up, glassing the C16's hull.  Another hurdle, but fun.

No, unfortunately, my 10 y/o MAS epoxy was not sealed, and furthermore was stored in a cold damp basement.  The hardenered turned a dark brown color, so I don't use it for clear-coating under varnish, but it otherwise works fine.

I heard a podcast by the owner of System Three who said that epoxy doesn't seem to ever go bad.  The MAS people told me to mix up a test batch, and if it cured it was still good to use.  So much for the warning dates.