About a year and a half ago we had our floorboards sanded, and the guy who did it filled the gaps with what I think was sawdust mixed with something. This looks fine but has fallen out in some places leaving gaps which are a bit draughty. I've tried filling some of them with brown decorator's caulk, which looked fine but started to fall out after a few months. I think this is because the boards are a bit creaky and move slightly, so over time the filling works loose. I tried brown silicone sealant (the sort used for sealing around window frames) in one place (it should be more flexible and hopefully might work better), but it looks a bit glossy. Does anyone know if i) using this sort of silicone is likely to work and ii) whether it's possible to get matte finish silicone sealant?