I see nobody has responded to your post, so I'll throw in my 2 cents. I used RAKA epoxy to make a copy of a historic Grand Canyon boat. When I sand with 125 grit paper, it turns white. When I recoat it looks great. I've applied final coats with a sponge roller and spreading it extremely thin. I have not tried sanding with finer grits, which might produce a satin type finish. I haven't had much of a cold temperature cure problem, as summer temps here in Phoenix have stuff going off after as little as 10 minutes of work time, but I can't imagine that the 610 hardener wouldn't be cured after 24 hours at 60 degrees. I found that the guy at RAKA is extremely helpful; you might give him a call... his phone number is on RAKA.com
What do you think the temperature was during the entire curing time? There is the possibility that it hasn't cured fully.
A paint job, whether it is done with a clear finish like varnish or a solid finish like paint will telegraph the condition of the surface through whatever you put on top of it. If you want a nice, shiny "Bristol" finish then you will have to spend some serious time prepping the surface before you add the varnish. In between the eight to ten coats of varnish you will need to sand according to the finish manufacturers instructions.
There are numerous discussions on the brand of varnish, the type of brush you use, the quality of your surface preparation, the temperature you need to have to apply the varnish and so on.
Even though I probably spent several hundred hours sanding my boat I could have done a better job and gotten a better finish, I applied a polyurethane paint so I would not have to repeat the process every two or three years. The amount of UV rays your boat is subject to will determine how long between reapplications of varnish.
You might be able to start with 120 grit everywhere but where you have runs or sags. Once you have completely sanded with the 120, move on to 180, 240, 320, 400 or even finer like 800 to 1,000 grit for a really spectacular finish. The smaller the "scratches" left in the epoxy the better, hence going in steps means that the finer sandpaper leaves the smallest scratches and reflects light better.
Google "Bristol Finish" to see what I am talking about. Only you can decide how nice and smooth you want the completed project to be. Please post pictures and keep us informed.
I don't know if you used raka but it will turn dusty when you start sanding. I made sure to let it cure well but I used the slow hardener for no less than 65 degrees. I'm talking 2-3 days before sanding. If you did go raka call them up and they'll set you straight. Here's an idea of sanded vs non sanded epoxy
Sorry just re read your question about sanding discoloration and bright finish. Yes, you have to sand through to a high grit to get a bright finish. I've never done it but I know that much is true. So it'll get darker before the dawn so to speak.