if you use buttercream icing and the best recipe i’ve worked with so far is the Wilton one - Buttercream Icing - then one of the tips is to use dip your spatula in a bowl of hot water, pat it dry on a paper towel and then smooth your icing with that. the warmth of the blade will melt the butter in the icing and give you a smoother finish.
another trick is the paper towel technique.
watch this video- Cake Decorating - A Secret to Smoothing Icing - MonkeySee
its very easy to do but i didn’t have the right paper towel so it left behind a slight pattern- d’oh.
unfortunately, i think Viva is a brand only available in the States. boo!