I used a 1/4 sheet Makita finishing sander to build a strip canoe, lots more sanding area than on a kayak. I now own a Porter-Cable ROS and am building a Yare. My two cents:

The ROS sander removes material faster. You should use it with the pad flat on the surface. If you sand with the curved edge (which removes material faster) cresent shaped indents will be created in the epoxy that won't show up until you varnish. If you are looking for that perfect finish keep it flat and sand more than you think you need. Sanding concave surfaces is easy with the surved edge but again you'll be creating those indents if you don't keep always keep the sander moving.

The 1/4 sheet orbital finishing sander will do a great job but take more time. I came up with a trick to use the 1/4 sheet sander on concave surfaces. Cut the sand paper 1/8" long in the longer dimension. Take a piece of 3/4" thick styrofoam and cut it to the size of the sander pad. Then shave the corners near the sandpaper clamps with a fine saw to form a curved surface on both ends of one flat side. Place the shaped styrofoam betweem the pad and the sandpaper. The longer paper will reach the extra distance. The styrofoam conpresses enough to fit the curves and it also tends to sand down with use fitting the curves better with time. It does tend to slide out from under the sandpaper if the paper is not taught. The other half of the sheets that are 1/8" short will still work in the sander for flat sanding.

You will also obtain a smoother surface if you use a cabinet scraper on the drips, sags, dust bumps, and foam brush bits. Using a power sander will remove material around the bump while removing the bump. You will find when you varnish that all of the bump, drip or whatever was not removed and a flat spot was created around it. All these little ripples in the epoxy don't seem to show up until the surface is shiney.

The two options to remove the ripples for the perfect finish is to 1) put on a sacrificial coating of varnish and sand after its cured or 2) my favorite wet sand (220 grit) after your power sanding.

Deep scratches will show as dark lines in the wood when the wood is wetted with epoxy. I used 120 grit on final sanding of the wood with good results, too fine of grit will redure epoxy penetration. I used 80 grit on the epoxy when more epoxy was going on, the new epoxy needs the mechanical grip and fully filled the skratches. Wet sanding by hand with 220 grit was smooth enough for applying varnish, any swrils left by the 80 grit will show through the varnish.

If you really want the ultra perfect finish you can polish the varnish after its fully cured. The question is: Do you want a museum piece or a boat?.

The amount of effort/perfection depends on your level of obsession or addiction.

http://www.greenval.com/FAQvarnishing.html

http://www.mcs.net/~brendler/oldtools/scraping/scraper.htm

http://www.taunton.com/fw/features/techniques/finish/1.htm