From your question it seems you desire a sheen to the finish. Acrylic deck stains have no inherent sheen. There is a good reason for this. It makes them slick when wet. You can apply an acrylic varnish and it will stick. Two problems. One is that clear finishes usually have poor performance and durability on exterior surfaces due to low solids and poor durability due to sun exposure. Second is a sheen added to rain makes for a slick and dangerous floor.