Inside walls - render skim?

hi all, I've had our rear outhouse tarted up over the summer (new roof, rewired) and to save a few quid I thought I'd have a go at plasterboarding the ceiling out and plastering the walls/ceiling myself, I've this week finished the ceiling which I'm more than happy with (after a bit of tickling with sandpaper here n there - lol) and am now about to tackle the walls, they're already rendered but the finish is a bit messy, and I wasnt quite sure best way to go about them, whether to re-render them or skim plaster all over! It's only a small outhouse building and were simply just tarting it up to use as another room, we had french doors put on the side to make it bit smarter and will prob put tv and some furniture in to chill in! So as the internal walls are a bit caccy and don't need to be proper smooth just consistent really, what's best plan of attack!!??!! Any ideas??