I have had hundreds and hundreds of aluminum wheels polished...by a professional polishing shop. Out of all those I have had a few done that I have welded up either small holes, or what not in. You can usually seem to see a faint outline of where the welding was done even after polishing. Even though they are pin holes now, once you grind them out with a die grinder and tig weld in..it'll be slightly larger of course.

I've had a bunch of valve covers polished too that were 60's or 70's era cast aluminum ones with the poor casting and porosity issues as well..never bothered trying to fill the areas and just left them as is since it was sort of just how they were. (and wasn't worth the cost to try and fix them all)

The same goes for porous areas in some old 5 slot wheels (Ansen, US Mag, etc, etc) just have left them as is as well. Did weld up holes that had been drilled for slicks before in Centerlines and some Welds and as mentioned those slightly showed through in some cases at least.

I use an excellent polisher and he can usually get out most pitting/porous areas on older stuff but the little that does hang around I have always just left. (don't leave any buffing compound behind though and won't have the black spots that way)