I was looking at the options I have for my Battle Smith's Infusions, focusing on the Replicate Magic Item, and noticed that one of the common magical items listed in XGE (P. 136) is called the Armour of Gleaming (which is regular armor that never gets dirty).
Given that this can be applied to any armor, that one does not need base materials to invoke this infusion, and that the Steel Defender has a Strength of 14, could I use this to grant my companion magically shiny plate mail?
You'd have to get buy in from your DM on whether or not you could make something for your Steel Defender. Nothing in the rules say you can't make plate armor that would fit whatever shape your Defender is. But there are some catches...
Some players change the shape of their Defender from time to time. If you do that, you'll also have to redo your infusions. ALL of your infusions.
There is no RAW way to stop an infusion, you can only create more infusions so that you exceed the limit and cancel out the oldest. So if you need to redo the armor, you'll need to redo all of your infusions until the Defender armor is cancelled out. This may or may not be a big deal, but it is something to be aware of.
They only have their natural armor. If you put on Plate, you invoke penalties:
Armor Proficiency. Anyone can put on a suit of armor or strap a shield to an arm. Only those proficient in the armor’s use know how to wear it effectively, however. Your class gives you proficiency with certain types of armor. If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.
So while your SD now has a better AC, they suck at ability check, saving throw, and attack roll that involves Strength or Dexterity. Is the trade off worth it? I don't think so.
