[MMI Enterprises]

Hey if that is good urethane (permenant solvent borne or 2 part catylized) not really meant for exteror wood decking your not going to make a dent with standard caustic strippers even if you boost them. Soy based solvent/gel offerings will get it with a good dwell time. Lots of money though. Better off sanding with swing machine equipped with the 36grit.
..good luck, urethane is a whole other animal! The 2 part stuff is what is also known as antigrafiti coating as standard markers or spray paint can be removed by paint stripper or oven cleaner without hurting it.