Removing Buildup on Dining Table?

I purchased a dining table at a garage sale. The table is dark brown. I do not know what was used on this table. I have tried vinegar, and ammonia and nothing seems to take off the wax or whatever was put on the table. When I try to put a finish on the table it is like it beads up. Without knowing what was put on the table do you have another suggestion? It doesn't matter what I try to put back on the table it does not take. Help!