Flexall filling of old cracked T-111 panels.

I am guessing you are referring to some vertical cracks?

Any pictures?

The cracks (checks) in T1-11 are usually all different directions due to the plywood face grain splits open with age. Use a mask if using/sanding it; http://www.dap.com/media/20795/00079912005english.pdf

IMHO, their "Plastic wood" is way better for your exterior use, I use it on my wood boat... ; http://www.dap.com/products/patch-repair/wood-fillers/

Gary
PS. Welcome to the forums! At least I hope you don't mean the joints at the panel edges...