Yeah, airflow and prep work really are the unsung heroes here. I learned that lesson after installing a small metal roof on my garden shed. Thought I'd done everything right—sanded, primed, coated—but I didn't pay enough attention to ventilation underneath. After one humid summer, rust spots started popping up from trapped moisture underneath the panels.
Since then, I've switched to using breathable underlayments or spacers beneath metal roofing whenever possible. Makes a huge difference letting moisture escape instead of getting stuck. Also, about those automotive-grade rust inhibitors... they're definitely pricier, but I've seen them hold up impressively well on some outdoor furniture projects. If you're planning something long-term or exposed to harsh conditions, it might be worth the extra bucks.
But yeah, no coating is bulletproof if water's pooling or trapped somewhere. Regular checks and keeping things spaced out for airflow is honestly your best bet in the long run.
Totally agree on the airflow point—seen way too many roofs fail prematurely because people underestimate ventilation. I've managed properties where the previous owners slapped metal roofing directly onto plywood without any spacing or breathable barriers. Looked great at first, but within a year or two, rust and corrosion were everywhere. Ended up costing way more to fix than if they'd just done it right from the start.
One thing I'd add: don't overlook fasteners. Even if your panels and coatings are top-notch, cheap or incompatible screws can corrode quickly and cause leaks or rust streaks. Stainless steel or coated fasteners specifically rated for metal roofing are worth every penny. Learned that the hard way after dealing with a tenant's leaky porch roof—turned out the contractor had used standard drywall screws. Not fun.
Bottom line, it's always cheaper and easier to do it right the first time, even if it means spending a bit more upfront or taking extra prep steps.
"Even if your panels and coatings are top-notch, cheap or incompatible screws can corrode quickly and cause leaks or rust streaks."
Couldn't agree more on the fasteners. I've seen a few roofs where the panels themselves were pristine, but the screws had rusted out completely—total nightmare to fix because you end up replacing perfectly good panels just to get at the damaged fasteners underneath.
One thing I'd slightly push back on though is stainless steel screws. They're definitely corrosion-resistant, but I've run into issues with galvanic reactions when pairing stainless fasteners with certain metal roofing types, especially aluminum. Had a client once who insisted on stainless screws for an aluminum roof, and within months we noticed some unexpected corrosion spots. Turns out the two metals weren't compatible without proper isolation washers or coatings.
Makes me wonder... has anyone else encountered galvanic corrosion issues like this? Curious how others have handled it in practice.
