My SS had alot of corrosion like that.
You'll want to physically clean out any aluminum oxide (usually white) for starters...I used a specific dremel bit that didn't dig deeper however went laterally through the alumina really well and even exposed "worm holes" that were buried beneath the aluminum surface. I didn't find any wormholes in the bottom skin however the transom was loaded with them.
Next, assess how much of the hull skin is compromised. I went around and poked aggressively to see if I can push through the holes with an awl or pick and even tried quantifying some of the deeper recess with a gauge.
single isolated pinholes got drilled out and riveted. Larger compromised areas I patched externally.
if it's a relatively shallow pit (most of mine were...consider superficial), you'll want to fill it with something to prevent water from pooling up in the little recesses. JBWELD or Marine Tex are fine.
To ensure that my filler adheres, I etched/cleaned the aluminum (phosphoric acid etch) then passivated it (chromate conversion). This is basically the Alodine treatment pros use for priming aluminum for painting. I used the West Systems 860 etch kit since i had it left over after a previous build.
