You can use gloss medium that you can mix with your paint while you're painting as well as a finish varnish that you can put over the top of your painting.

The acrylic medium is water-resistant but does not provide a waterproof coat. To make it watertight, add a sealer over the acrylic paint. Although it might be slightly water-resistant, it does not provide a waterproof coat. To make it watertight, add a sealer over the acrylic paint.

Acrylic gloss medium This medium increases the fluidity of the paint whilst simultaneously providing a solid paint film. This medium by Mont Marte can also be used as a varnish as it dries to a clear hard surface.