There is an issue with many of the "newer" radiators.

The picture indicates corrosion in the cooling fins between the tubes. This is common in AC units as well as autos where there is a high salt content in the air; like when the unit spends most of its time within a mile of the coast.

The fins are aluminum and the corrosion is due to exposure to high salt content of the air.

There are (or were a few years ago) radiators that did not use aluminum fins. Of course, they are a little more for one than you would pay for two of the common variety.

I used to work in a Navy shipyard and our AC units were replaced annually due to salt corrosion. I went through a couple of radiators before finding one that was not cheap aluminum fins. Now, I have no problem as I no longer work at the ship yard. They still have AC problems but my radiator problems are solved.