I always use Jo Sonja's All Purpose Sealer for this. You can use whatever you are comfortable with.

I have also found that thinning the sealer 1:1 and putting on about three thing coats is much better than one thick coat. You end up with a much smoother surface.

Always brush from the center outwards to the edges. The reason for this is to reduce drips along the edges. When you brush from the edge inwards it is like wiping your brush at the edge of your paint can. It just squeezes the varnish off the brush and it drips down the side of the board or box.

Allow each coat to dry according to the instruction that came with the product ... now to the next step ... Varnishing.

Using a Spray-on Sealer.

You can also use a spray on sealer which is the easiest way to go, but check it on a spare piece of print first because occasionally, you find a product that does weird things to the print such as making the colours run or making the paper turn transparent looking