hire a large floor sander from hss for the faces of the boards, use 80 grit discs on a powerful r/o sander for the rest, and loads of 80 grit paper for the nooks and crannies, make sure no protruding screws / nails, there u go..... one lost week-end! ebanista
Find a slat of wood that loosely fits the groove, and use it as a thin sanding block in the grooves. If you want to do it all by hand, or hand-tool, think about wrapping sandpaper round a worn-out broom(need a strong handle)for the large flat areas. Staple it on at the top side of the foot. Won't be as good as on your knees and in your hand, but will save your back some. Mr. Handyandy - really
If you want to get rid of the varnish both in the grooves and on the flat top section you might consider getting it sand blasted. One of my neighbours got a contractor in to do this last year. A couple of hours cleaned up a huge area of decking. He then ran over it lightly with a DIY grade 1/3 sheet sander to get a bare foot friendly finish on the flats. He had estimated that it was going to take three weekends to do using a floor sander and then hand sanding in the grooves but it was all over by lunchtime. Graham