I’ve got a gap between my chimney and the surrounding framing that I need to fill with fire block foam.  I checked with the building inspector and he’s cool with it because the chimney doesn’t get hot (*rimshot*).  The gap is around 3/4 in wide and 6 or more inches deep but obstructed in places with old chimney debris and uneven brickwork.  And of course it’s vertical.

Any tips on how to fill it effectively?  I want the foam to expand evenly and form an effective seal.  What makes this more difficult is the fire block foam comes in cans that won’t work with a foam gun so I have less control.  Do I need to apply it in layers?  Stuff the top of the gap with mineral wool?