I've never heard of that specific brand, but those kind of general (not specifically made for plastic models) hobby paints apparently often aren't all that great for plastic models since they tend to have larger, coarser pigments. I have no idea if that's the case with your specific one since I've never used it, but it could be. There's a pretty good list of scale-modeling paints on reddit (
https://www.reddit.com/r/modelmakers/wiki/index#wiki_painting). If you can, I'd recommend picking up one water-based acrylic from the list (from personal experience, Vallejo Model Color and Revell Aqua work great) and seeing if that works better. That + primer should give you better results.
I did look up a some youtube videos of people using the paint and it definitely seems way too thick straight out of the bottle (
example 1,
example 2 - both have very thick paint that leaves behind very visible brush marks). It could maybe be that the paint just wasn't mixed properly. Since the paint is old, the actual paint pigments have likely settled at the bottom while the thinner remains at the top, which would explain why the paint is watery and is sliding all over the place.