You can certainly use fillers as well, I just find grout works for me. If you decide to mix your own, I would recommend adding p a and water to dilute. Mixing your own is cheaper but I use mine for bases as well so I use small quantities and it became a faff mixing a small batch so I switched to the ready mixed. Inevitably some goes hard when you get down the tub but it is not expensive. Incidentally, stay away from Tetrion, it is impossibly difficult to carve once set.
John