Most likely that needed a catalyst and what you have there will never dry, cotton rags soaked in reducer should wipe it off.... Get it wet, and wipe it dry with the other hand (wax on wax off kinda thing) and work across the piece in small areas always working a new area.... It will be a sticky mess but just move on. get to the end start over on the top, around the 3rd time it start to clean off without soaking into the MDF, let dry and one more final clean, don't let the reducer saturate anywhere for very long, It will stink use ventilation, and will require many rags and around 1 gal, of thinner.