Acrylic paints have different sheens, and these change further with the addition of retarders and clear extenders. A finish or sealer gives an overall sheen that does not distract from the work itself. One wants viewers to notice the art as a whole – not dull and uneven areas. A finish coat also does help to give more depth to the color.

Second – The chemical nature of acrylics means that dust and dirt not only is attracted to – and clings to the surface, but will over time become one with the surface. The varnishes and finish coatings made for acrylics give a harder, slicker surface so that less dirt accumulates. They also allow for cleaning the paintings in future without disturbing the paint itself.
Patrice Lynne Young´s last blog ..Three small paintings…

I’ve used a spray fixative that gives the painting a bit of a glossy finish. It’s quick and easy and pretty effective.

I’ve also used what’s called galkyd medium. It’s a clear, oil-based medium that gives paintings a very shiny, wet look. It really enhancs your colours, but like oils, takes a long time to dry.

There are other ways to seal acrylic paintings, but those are the ones I’ve had experience with!