After roughening, a fine sanding dust remains — and dust acts as a release agent that ruins the glue joint. Brush the whole surface off and wipe with a clean dry cloth. Avoid wet wipes or water here. The test: run a clean dry gloved hand over the surface — if you feel no dust you are ready. Always use a glove; bare skin leaves grease that weakens the grip. When dust is gone, wipe the contact surfaces with technical spirit (isopropanol) and let it evaporate fully.