When should I use shine spray on an acrylic painting?

Use clear acrylic sealer. It comes in gloss or matte finishes. Its never too late to spray a sealer over a picture. The spray will not only bring out the true colors of the picture but will make it more permanenet. You can buy the varnish at any craft or art store or wal-mart in the carft section. And the acrylic spray will seal anything or go over anything including old varnish, or shellack or even oil paint , watercolors ,pastels, charcoal , etc. It doesn't matter. It will seal anything.