Service Complaint

I bought a wooden garage door in March 2019, it was installed towards and if March. The door is now separating in between the design joints. They say they won't fix or replace because I painted with wooden oil and vanish. If it was not taken care of they would have said rain damaged it. It's only two months they installed the door. The cost was R13 000. I feel ******. I need help to resolve this. They say i must pay R850 for them to fix the door.