Hi Matthew,
I tried a couple of types of small rollers, close cell foam and a small nap 4mm "urethane" roller, results were similar and probably as good as you will get using a single pack polyurethane and roller/brush finish [spraying will give a smoother finish] I don't have spray equipment. Both Norglass/International state that prep is 90% of your painting time,yup, your undercoat/primer can't be too smooth as the topcoat will highlight any blemishes. The can states up to 5% painting thinners to help it flow, the website says 3%, I did get some tiny bubbles so maybe mine had a bit too much thinners, plus I had a warm [35C] weekend when topcoating,
Hope this helps,
Roy.