Hi Rate1
You could try one of the UK stockists:
http://www.inwoodmodels.co.uk/system/index.html
goto the drop-down buttons:
materials>>>Glue>>>ZAP
to find the Finishing Resin (+ 5 and 30 min epoxy glues CA's etc).. I'd be surprised if your local model shop can't get it for you...Inwood will charge you postage at cost.
If you're going to use your boat resin for sticking your balsa sheet to the foam cores unthinned you will add an awful lot of weight.. Again I have only ever used thinned Zpoxy finishing resin for this task...you can test this theory by painting some onto a small piece of polystyrene foam and slicing it open when dry to see how far the resin has penetrated...hopefully not at all.
iirc, Sig used to sell a bonding adhesive (Sigbond??)...operated somewhat like Evostick where you spread the stuff on both your cores and skins and then put them together after the glues has dried to the touch....problem with this method is that you only ever get one go so your positioning has to be ok first time. Other folks I know have used 3M "spraymount"..a similar aerosol adhesive for making posters etc. I also know of people who have failed using this method with their skins becoming unbonded..
Another suggestion is to use the ProBond suggested by Mick Reeves for bonding his Proskins...that stuff sticks like the proverbial and you may also get a few secomds to get it positioned OK.
Just a few ideas to get you thinking...the boat stuff will be fine for repairing your Lotus Elise or Esprit.
More Anon