In 1985 I sanded and repaired coachroof to wood, lightly stained it, gave it a coat of SP resin, then fibreglass scrim, 2 more coats of resin then 5 coats of 709 2K Polyurathane varnish, last two coats sanded before applying varnish with 320 paper in a DA sander.
Apart from considerably adding to it’s strength it has been maintenance free to present time. All coats were done within 24 hours of each other to ensure a chemical bond.