Sponge, if the rocket you are applying decals was painted with a flat paint then I would first apply the glossy clear. OR, if you used glossy paints I would apply the decals and go the <a href="https://www.rocketryforum.com/showthread.php?s=&threadid=9331&highlight=Future">Future Floor Polish</a> route. Future will futher help your decal be laminated to the rocket and may also prevent the self-stick from lifting away from the rocket if ever an edge catches the decal.
Another good tip for the self adhesive decals is to dip them in a saucer of water that has a drop of washing up liquid in it.
This allows them to slide around a little, like a water slide decal. I admit that when I first heard this I was a bit sceptical, but it works, and doesn't appear to have any detrimental effect on long term adhesion.