I am a little surprised that Q cells would be used as a thickener at a hull/deck joint, but if it is strictly a filler and not structural I'm sure it's OK. Q cells are usually used as a lightweight filler for sanding
and fairing. Being microballoons they "fluff up" the resin, increasing volume, often as much as double, as it thickens. This makes the mixture very light weight and easy to sand. A stronger choice would be a fumed silica thickener like Cabosil or West 406. These thicken the resin without adding much volume.