I have a rust hole (okay, many rust holes) that I plan to patch over. It is not visible so I will just patch, prime, and paint. I have ground down good metal around the rust. Is it still necessary or better to remove the rusted metal?

Thanks.

Remove it.

The rust you leave will come back to haunt you!

Thanks.

The responses have spurred me to get more information.
I have done some reading and research and have a better understanding now.

It is not so much that the rust will spread. Technically, it does not. However, metal must be sealed against the elements to prevent the oxidization so the affected areas need to be cleaned up and sealed, then patched. The rusted metal will be porous and, unless it is sealed, I am guessing this is what the rust converter products promise to do, moisture will penetrate and more rust will occur.

Next question, if patches are lap welded into place, the metal has to be bare and there will be at least some metal behind the patch that will not be sealed. Additionally, the back of the patch will be unsealed at least around the edges. Inevitably, moisture is going to get into some of these areas. How can this metal be protected?

Thanks again.

Yes, it's still necessary to remove the rusted metal, even if the area isn't visible. Leaving rust, even under a patch, can cause it to continue spreading and compromise the integrity of the surrounding metal over time. The best approach is to cut out as much of the rusted metal as possible and then patch over the clean, solid metal. Once you've patched, make sure to use a rust-inhibiting primer before painting to prevent any future rust from forming. It might seem like extra work now, but it will save you headaches down the road by ensuring a longer-lasting repair.