I'm making some mirror surrounds from oak, ash and beech. All 25mm thick by 120mm boards, I need to glue and clamp 2 boards together so I can route out a to drop the mirror into. Problem I have is the glue, I know if I use ordinary PVA wood glue I'll get a dark line at the joint which I don't want. So whats the best glue to use,preferably usable with biscuits, that will be invisible even when oiled. Thanks.