Clear Coating

To Clear Coat or Not To Clear Coat - That Is The Question!

I Say Ding Dong stickers do not need clear coating. We have stickers over 10 years old that have lived outdoors on vehicles all that time and are still fine. Yours will be too  unless they are to be subjected to hostile environments or aggressive fluids or pastes. This includes most modern polishes and screen cleaners as they are solvents as well as the obvious one - Petrol. This is why all of the manufacturers clear coat over stickers applied to the petrol tank. Naturally the stickers durability &  lifespan will be increased should you choose to do so. But Clear coating can be a minefield.
We are not professional painters so cannot give definitive advice on this however this is what our customers tell us:
Always try the compatibility of the clear coat you wish to use on an off-cut of vinyl the same as the sticker you wish to cover (we will happily supply some off-cuts with your order if asked at the time of purchase.
Clear coating always requires lots of very thin coats, a great big heavy dollop of an initial application will nearly always cause trouble.
Some tell us PPG or BASF type clear coats work well if correctly applied.
Urethane can work well if applied professionally.
Never use cans of clear from Halfords or similar, experiments with these seem nearly always doomed to failure.
Talking to the experts they tell us that impatience, poor prep, unsuitable materials, too heavy coats and  common or garden lack of skill cause the most problems. 
But even they admit to still having issues occasionally!