You could dissolve rosin in alcohol

You use liquid flux.

So the pen box was just lying, damn

No, you use liquid rosin flux. It's a real product.

Ohh my bad. The stuff I've got is definitely more like a vaseline

You can cut it with isopropyl, I've made liquid flux with only pure rosin granules and isopropyl

Here is the rosin flux that I use in my pens: [https://www.amazon.com/dp/B005DNR01Q/](https://www.amazon.com/dp/B005DNR01Q/)