There are clear lacquers available in paint on or spray cans, matt and satin, we discussed it a few times over the years as some are ethanol proof and some aren't.
You will need a varnish that won't react with the paint type you have used...I usually use a brush on varnish and just slightly overlap the edge of the decal to 'seal' it....If you do use a brush ensure you have a good quality brush for a smooth finish..I use small watercolour brushes...Ian