you are at the low end of the temp range for that paint.
I WOULD use hardner otherwise it will take forever to set.
as for the roller.. um.. ok... I prefer spraygun myself.. but.. whatever you gotta do to keep the rust down.. I guess..
I would primer it too.
ps... for hardner.. hit a nason dealer and get ther quik ure hardner for their single stage syn enamil. much superior to t he valspar hardner IMHO..