I have yet to launch a capsule into space built with homemade castings.
Therefore extreme concerns over weight don't apply, and my aluminum castings are multiple magnitudes of degree heavier than absolutely necessary for their intended purpose. It's frankly a pain to cast too thin a piece, and mass and solidity benefits most machine parts I've ever made as well as looking more substantial and aesthetic.
Occasional tiny pinholes requiring a magnifying glass to see in an aluminum casting are not therefore viewed as a problem. Now, I'm all in favor of others taking steps to perfect their casting, don't get me wrong. But I cast (as most other home shop sandcrabs do) with scrap, which is not devoid of corrosion, oil, dirt, or included steel clips (pistons, my favorite source of aluminum always has steel cast right into it). So the concept of avoiding contamination is doomed from the start. Luckily, it doesn't matter -- the contaminant either burns away into carbon dioxide or powdery dross (which actually protects the melt from oxidation), is fished out (clips) or skimmed off (dirt, oxides etc.) before the pour.
I did once put a crucible cap on an iron melt. Didn't seem to make any difference.