Various paint manufacturers sell fluid acrylics, but if it’s only something you’re going to want occasionally, you can make your own version from your usual, more buttery acrylics (it works best if the tube of paint you’re using is artist’s quality and soft-body). Here’s how to do it: A dispersing medium would also work, but check the label to see how much is ‘safe’ to use. With some, if you use a lot, the paint may become water-soluble which could be a nuisance when applying further layers of paint. Another option worth considering is to use an acrylic ink rather than paint, as these have a very fluid consistency and intense colors. Watch Liquitex Pouring Medium and Using Liquitex Pouring Medium, by Michele Theberge to see how to create a resin-like coat on your paintings.
