I just bought a wood grain, dark brown color fiberglass front door from HomeDepot. The HomeDepot guy told me to put a coat of “varnish” on the door but a painting contractor told me not to and just leave the door as it is because the varnish will wear out

Paint keeps cracking on interior wall. Scrapped off paint and re-plastered the wall, waited a week and the same thing has happened. Can any one help?