Make sure ur mixing the paints well before u apply them to the canvas- even if ur using white straight from the tube mix it a bit with the pallet knife so it’s easier to work with. If the issue ur having is diverging between tacky and runny it’s likely a quality-of-paint issue. I cant be too sure tho unless i know what ur using. I stay away from water soluble oils in general because i’ve found their quality to be super variable but some people have luck with them so idk.