Chances are you will need a thicker layer than 2mm for a Nacra hull socket. I've had one socket on a Nacra 20 require over a cup of resin to fill the voids in it. I've done about 6 or 7 various Nacra beams including your F18 model. Seating the beams does make a noticeable difference the the boat stiffness. You can use gelcoat or thickened epoxy to seat the beams. The gelcoat is nice in that when some squeezes out the edges, it should match the color of the boat and won't stand out - you don't need to be as cautious with it. However, I've used thickened epoxy on all the beams I've set previously but will probably switch to gelcoat on the next set I do. I first use clay to make a few "snakes" and use this to wrap around the beam bolt holes to prevent any adhesive from making it's way into the threads there. The clay will compress and make a dam when you snug up the beam. I also put a clay snake around the lower exterior sides of the socket to keep anything from oozing out where I don't want it to.
The waxed beam process is the same as mentioned previously and works to let the beam release. Use a good automotive carnuba based or specific mold release wax.

I thicken epoxy with cabosil (fumed silica) to a creamy, sour cream-like, thickness and pour it in the hull socket and work it around a little. The gelcoat should work in the same manner. Then I set the waxed beams in place and snug up the bolts...almost tight but not quite. Sop up the excess resin/gelcoat as it squeezes out (use denatured alcohol to clean up the epoxy, use acetone for gelcoat). I let it kick like this, periodically checking and cleaning up weeping resin, before tightening up the bolts the rest of the way.

PS - DO NOT use 3M 5200 as a seating compound. Besides not being a rigid material, I helped a friend try to remove a beam that someone had set this way...after trying everything for two weeks, he finally got it loose but not after some significant fiberglass damage to the beam socket and deck.