I use it without a barrier over acrylics all the time, as that is how it's really designed to be used. Their paint sets are acrylic and their weathering sets are enamel just for this purpose. It's only when you use enamel paints that you need a barrier.

There are no instructions with the products, true but on their site there is a link to 'techniques'. You may have to search back through older posts to find exactly what you are looking for but there are lessons on most, if not all of their products, mostly done by Miguel himself.

http://ak-interactive.blogspot.ca/