I repair alloy wheels on cars, and tbh most of the porosity that causes air pressure loss is due either to corrosion occuring inside the rim where the bead of the tyre seals against the wheel, or from the valve core not being tightened properly when a new tyre has been fitted.
I've used such puncture sealants before, and in all honesty, it coats the treaded area of the tyre, and keeps well away from the sidewall and the wheel itself.
HTH,
Paul