Originally, artists felt that acrylic paint was such a durable medium, that now top coat was needed.  If the paintign got dirty, just was it off and it was as good as new.

A few artists felt that a barrier coat of acrylci medium should be added to further protect the painting.  If the painting requiere more aggressive cleaning, the coating would be abraded and not the pigmented surface.  A new barrier coating could then be applied.

This became common thought about five to six years ago.

Recently, Golden Acrylic hosted a conference of chemists and museum curators to discuss this topic further.

By the end of the conference, the recommendations that had been formulated said to add a second coat of mineral spirit acrylic (MSA) over the barrier coat of acrylic nedium.