That's pretty normal separation for any oil or polyurethane paint w/ solids in it.  Yes, you can and should very thoroughly mix the paint before use.  Tip: If you know someone at the neighborhood paint or hardware store, and you have several cans, he might put them in his shaker for you, speeding up the process a lot.  They can take many minutes stirring by hand, and you slop it all over your hand.  Bring him a cold drink!

If you didn't mix the first can well, the stickiness could be because you were painting with mostly the oils and resins, without enough of the solids to make a solid coating.

BTW, I used PreKote on the outside of my skerry hull, partly for its high build properties to hide some of my sanding errors.  I think it creates a softer base for the Brightsides finish coat that if I'd just painted Brightsides onto the sanded epoxy.  My outside hull seems to scratch much easier than the inside, which is just Brightsides on the epoxy.  No scientific tests, just my impression.  If I do it again, I'll be more persistent in getting a good epoxy base for the finish coat and forgo the primer.  

Can anyone speak more on the suitability of applying brightsides paint directly to epoxy? I may have to use a paint scraper and sand paper to remove the gummy mess from inside my hull, so I am a little wary of putting the same product on the outside. I have a fairly smooth finish already, and I am not worried about making it flawless.

I think first can of Pre Kote that I used on the interior of the hull was well-stirred. It had not separated nearly as bad as the second can I opened for the exterior.  

   Epoxy is an excellent substrate for paint. Unless your epoxy has amine blush on it paint will tenaciously adhere to it. Use of a high build primer is really just the final fairing of a surface before painting. Most of it should be sanded off and large swathes of naked epoxy are no problem as long as it smooth enough for paint. High build primer does not help paint adhere to epoxy any more than applying the paint directly to the naked epoxy. If you can get a properly smooth surface with epoxy alone then you do not need HBprimer. But that's a big if, DAMHIKT.

In my experience as a Peeler Skiff builder, Brightside polyurethane paint adheres beautiffly to a properly prepared (cleaned and sanded) epoxy substrate. I used Brightside over both plain epoxy and over Pre-Kote (on epoxy). Pre-Kote's only purpose under polyurethane is as a high-build promer yo fill imperfections. It is adversly affected by moisture in the air and must be applied and covered by paint while humidity is low. I probably will not use Pre-Kote on my next project and spend the extrat care in surface preparation.

As with any high-solids formulation, Pre-Kote will settle in the can and must be thoroughly mixed before use.

Cheers,

Dick