There should be seal around the inside of the bottle top, it could be missing, if it is missing or it didn't have one, cut a piece of hard cardboard to place over the bottle neck so you can shake without it leaking, you might even have a solid bottle top on a different type of bottle that fits that you can use for shaking, I have golden high flow too, when I shake I put one finger on the top of the twist cap and push down while I shake, thstcwoukd fine for me, you might also have dried paint somewhere inside the lid that's stopping you getting a goid seal when it's closed.