For me, I don't do sanding with water/detergent with wood, even if it's marine plywood pa, I don't like the feeling of my wood absorbing moisture (and especially water for that part). Dry sanding lang kahit na matrabaho, 80 grit first para mabilis, then 220 and higher to smoothen and remove scratch marks.
I leave the wet sanding for automobile/metal use.
Also, I think the putty is not a good idea, kahit pa wood putty/filler, mahahalata kapag na-stain na (well, to my eyes actually..). Putty for me is best for applied for duco finish or leather-wrapped enclosures.