Trim paint peeling off- what should I do?

[Sam Zawatsky]

Hello,

I am painting a house for a family member, as the previous painter that was hired to do so was cutting corners and not doing a thorough job. As I was sanding everything, I noticed that the newest coat of trim paint that the fired painter put on without sanding (Sherwin Williams proclassic satin interior) did not stick at all to the old yellowed door casings. Whenever anyone even Lightly scratches the door casings the paint begins to peel off and can’t really be stopped until it is all scraped off. What should I do? I was considering scraping/ sanding off off all of this recent coat of trim paint and aggressively sanding the old yellowed paint so that it will accept the new coat better. Will this be enough? Do I need to sand aggressively, then apply a special primer or something before the new coat?