Here's one thing I know of: http://www.minwax.com/produse/woodrep/puwrwh1.htm I have never used it but I have heard it is good. Here is their filler: http://www.minwax.com/produse/woodrep/puwrwf1.htm It is basically bondo and I think it is only any good if you are going to paint it afterwards. If you aren't going to paint this wagaon then I would replace any large pieces with a dutchman. This is a piece of wood you cut and carve to fit in the void. Good luck, remember the old wood has to be 100% dry before you do any of this stuff to it.