I think it's easier for a watercolorist to switch to acrylics than an oil painter.   You're trying to thin out "heavy body" acrylics. They're fine for impasto or knife painting, although too shiney for my taste. You're also working on a non-absorbant surface as opposed to watercolor paper.  First make sure you're not using an oil-primed surface. Then I would suggest using liquid acrylics, and a thinning medium of a little retarder and water. I know at least one manufacturer that makes a sample set if cost is a factor.  You might also try sanding the canvas very lightly, and with a very fine sandpaper.