If you're doing the second coat the next day, no sanding is necessary.

The CLC fiberglassing FAQ says you should do a light scuff sand if more than 30 hours have passed. As explained at the Okumefest seminars, "light scuff sand" means give it a pass with green ScotchBrite.

It also depends on the brand of epoxy, the hardener speed and what the temperature's been like since the first coat was applied. Chenier's answer is perfect for MAS with slow hardener at comfortable temperatures over 60 degrees.

The best thing to do if your setup is different is to check the epoxy manufacturer's data (usually available on their website), but a good set of practical rules are:

1. If it's tacky, no sanding needed

2. If it's not tacky but you can still make a dent with your thumbnail, no sanding is needed

3. If the sandpaper, ScotchBrite or whatever is getting clogged with little gummy balls of epoxy, stop sanding and apply the next coat

Have fun,

Laszlo