Me, I never mop butts. The only time I ever sprayed them with AJ was when I had to go in there for something else.. (just my $.02
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Since you're doing 2 butts, may I suggest you try both rubs? I've done a boatload of butts with Texas BBQ Rub #1 and #2 and personally, I like the #2 better. That's some good stuff!!
Thanks for the advice ... I was going to put one butt on each grate ... if I used two different rubs would I have an issue with the top butt contaminating the lower?
I searched previous discussions and for a mop I was leaning towards something simple ... either straight apple juice or AJ mixed with oil.
I think it would be minimal if at all. I've done 4 butts with different rubs on each level and haven't noticed any. Both butts will fit on the top grate without touching each other AND they'll be at the same temp too.
I just add some of whatever rub i use (and turbinado sugar, for butts---of course, I will have rubbed that on the butts too) to some apple cider vinegar and sometimes some apple juice or cider if I have it, and mop with that.