I'll get you started and then others and I will chime in. You are basically taking on a big job and one that will make a mess of your house and floor.
Start by removing a door off the hinges and take it to the garage and apply some citristrip per instructions. Then we can go from there with what you find. You will further be best served by removing the baseboard trim and door casings and just as well remove the jambs and work on them in the garage. They are easily reinstalled. Do one room at at time. I explained below in another post my easy way for removing trim and jambs, etc. Post back when you get that far or can't find it. Good luck.