I recently played a game with Ashe, and did VERY well farming. I was 5-0 with about 150 minion kills, and was 4-5 levels ahead of the field (11s to 15 or so).

Things then took an immediate downward spiral when their whole team bought thornmails. How does someone like ashe, who is PURE AD solve the issue of killing herself from thornmail. I had a fully stacked BT and my original doran blade from teh beginning of the game at that point.

I started building a 2nd BT, but is this correct? What other things could I look into if i run into this situation again

You cry quietly in the corner and hope your AP carry shows your foes the error of their ways.

a second BT is a good choice but that solves the reflecting damage part but not how much damage you will be dealing. The extra 100 armor makes it incredibly hard for you to kill anybody so you need to get armor pen such as BC or Last Whisper. If they have over 100 armor this is extremely useful. Your health should be relatively safe if your not focused because of the life steal from your first BT but if you want to continue dealing damage get armor pen and then you can still try to get some kills and deal damage.

Have your ap carry wreck their whole team then, they'll all QQ and be like start build **** loads of mr.