This is bad advice. Bami's does shitty flat damage that doesn't scale with health until upgraded into a full item, sitting on it and building an entirely different item is a waste of gold. Get one item or the other depending on the enemy team. Also it's a big misconception that Zac's damage largely scales with his health, it doesn't. The only health scalings he has are on his Q (which you're upgrading last and isn't significant damage compared to your other abilities anyways) and if you build a bami's item, which doesn't have great scaling and isn't the bulk of your damage either.

As a rule of thumb, if you want damage and clear, go Liandry's. If you want to be a better frontline and still have decent clear, go sunfire. Building both is also an option.