Old paint that has undergone wear and tear will need to be removed when you’re applying new paint.
Peeling and Blistered Paint
These need to be stripped with a paint scraper and evened out. Alternatively, you can remove the whole paint job in its entirety.
Loose Paint
These need to removed with a scraper as well. If they leave flaws, you should apply a coating compound to level the coat or sand them to an even finish in preparation for a new paint job.
Badly Deteriorated Paint
You can try scraping it as well. However, if that doesn’t work, you can use a heat gun to get it to come loose or a chemical paint remover, easily available from most hardware stores. The chemical paint remover should be applied to the surface of the old paint and allowed to dry. After that, it should soften up the paint coating enough to pry it clear.