@N8man I believe that if the barrel is burned hot enough you should not have a problem. The problem is that like in my first burn the lower 3/4 of the barrel got hot enough to burn the exterior paint off, but the top 1/4 did not get that hot. Thus the reason I still used the weed burner.
Now with all that said, since one may not be 100% certain that the entire barrel was heated to a degree that will sanitize/neutralize any harmful toxins in the barrel , it would be prudent to start with a food grade barrel if possible.
Luckily I found food grade barrels, but if I did not, I would have still used them and taken the same precautions. Besides, as I said before; just because the barrel originally was food grade, it could have had a second storage use before we got it.