I have an older windsurfing board with fingernail size crack in the bottom. It is a small but very deep crack that likely goes all the way to the core. I don't need a fancy repair as most of the board is covered in aqua need already anyway. Which of the above do I use to make sure I don't melt core? Which one is the least viscous? I am going to need something that will flow down through the small crack rather than pool at the surface. Thanks in advance for any recommendations.
If I was desperate, I would use Araldite. The other two products I have no experience. The preferred product for board repair is West System.
If you want to get the compound inside board, then use a syringe. But be careful, as there is a technique to this. It's easy to add bubbles when sucking compound into syringe , and if injecting syringe into fitted hole (4mm), compound will be under pressure, and will leak/spurt out (so need to leave syringe inside until pressure equalises). I normally form around the hole using blue tack just in case this happens. If you get it right, the repair is perfect 4mm repair and just a small amount of sanding to make the plug level with board. Then paint. This takes a bit of practice to get it right, but I find this is a very ruggedised and minimalist technique for very small repairs.
Good luck 
UV won't cure to the bottom if the crack is that deep.
Not araldite as its not designed for continuous immersion (but OK for an old board it prob wont matter)
But sounds like its so minor it does not need repair to the very bottom....
Go here and watch last video. www.seabreeze.com.au/forums/Windsurfing/General/Board-Repair-Videos?page=2
Only change I would make is scrape the crack out with a scalpel or box cutter blade so its more like 3mm wide and 1mm deep vee that you fill, forget getting the resin to the bottom of the hole - UNLESS it is large and in a structural area like the middle half of the board and the board is used hard (jumped etc)
