When getting Artificer Armour and rolling for Histories, if you roll on the table to choose Power Armour histories and get a 4, that's the Skill of the Artificer table. The only thing is, literally every option on that downgrades the armour, even Mk8 chest, usually by 4 or 5! While the last four options do provide other benefits, they all reduce either the arms or the head to Armour 7, rather than the Armour 12. Would you guys say it was fair to either reroll which table to use, or to say that, if you get one of those results, the armour has been reinforced and upgraded to the point it grants Armour 12 anyway? So, while you might see some benefit, say out of Mk6 Greaves or Helmet, or Mk8 Gorget, the only time the marine would see a real benefit is if he got an upgraded form of Histories 7 through 10, in which case he wouldn't see the downsides because of the very fact his armour is an Artificer suit?

keep in mind those armor histories are specificly noted as being "Examples" and that "DMs are enchouraged to be creative" so there's nothing wrong with saying "those art artifacer mk 3 greeves with an armor of X and these other benifits due to the extremely high craftsmanship of the legs"

in short, as a DM make something up. for artifacer armor specificly I think the idea of "these vambraces and gloves are extra well articulated thus removing the penalty to fine movement" etc is fair

I would to it this way.

If your player has an Armor 12, an than he rolls a History that reduces Armor from statistical 8 to 7, than I would reduce it by 1. So if AP8 becomes AP7, than AP12 becomes AP11.

That's how I'll do it once my players hit Rank 7 (in around a week or two, I think, unless they all die heroically instead).