With the interior roughed in, it was time to start putting on the finishes.  When I bought the frame and trailer, the previous owner had a stack of old pine boards off an old barn.  Some were floor boards, some were siding, some were painted, and most had quite a few nails in them.  I had originally thrown them out of the house and considered them scrap wood, but when I started to look at flooring costs, I decided to give them another look.  I sanded one down, put some stain on, and it looked amazing!  Flooring found!  They needed a lot of work though.  First, I spent 3 hours carefully inspecting each board and pulling out every single nail I could find.  Then, I borrowed a planer, and planed them all down to 3/4" thick.  Some of the boards only took one pass.  Many of them took 3, 4, and even 5 passes, so it was a very time consuming process.  With the planing done, I sanded each board with a belt sanded with 40 grit and then 80 grit sand paper.  Then it was on to a handheld orbital sander with 80 grit, 150 grit, and finally 220 grit.  Next, wood conditioner, stain, a coat of poly, sand with 220 grit, coat of poly, sand with 220 grit, and a final coat of poly.  Very time intensive, but I am extremely happy with the results, and the price was right!  I installed the bathroom floor first and then moved on to the main floor.  These were all previously used boards, so the width and length of each board was different, and most of them were not straight.  Needless to say, installation was a challenge.  It was part jigsaw puzzle trying to find pieces that fit and part brute strength manhandling them into position.  It took the better part of a day to put the floor in, but I am very happy with the results!