Ain't long boarding fun? The necessary sideways upper body motion calling heavily on otherwise little used, easily tired lats, arm and stomach muscles. Learn to love the 'burn.' Oof!
Smoothing the relatively soft foam is the easy part. After you 'glass the outside of the foam you'll be doing more, a whole lot more; initial glass and epoxy layer(s), filler coat(s) -- do not use 'micro balloons' or other lightweight fillers, final epoxy-only top coat. The surface will be harder, you'll be using successively finer grits, down to 320 for sure, 600 maybe.
I recommed that you consider a pneumatic long board. Nowadays, it is more often associated with automotive body work. Something like this,
From
onlinetoolsupply, 'only $90' if you have access to a compressor of sufficient capacity, volume more an issue than pressure. (Don't worry, you'll get used to the post-use buzzy, pins-and-needles feeling in your hands, its not permanent.) Also, it'll be cheaper to buy bulk rolls of abrasive strips (automotive body shop supply) rather than packaged precut to length (Home Despot, etc).
[Edit] Come to think of it, when you get to final sanding the last filler coat and epoxy topcoat, the preferred tool is a light weight, pneumatic circular random orbit sander known in the trade as a 'jitterbug.' When used with a flexible rubber supply hose, vice the more typical stiffer, cheaper, hard plastic hose, it is much more resonsive, gives better 'feel' or 'touch.'