sanding planking

There is almost no way to do it in any expedious manner. Go with a fine grit (180-220) in the proper direction on the ribs and don't worry too much about the planking. A lot of folks will use sanding stars in a drill. If the grit is fine enough, it won't show the cross-grain sanding under a few coats of spar.

Good luck.
Here's a couple/three ideas. None of them is painless to work with.

Dremel contour sander with the detail pad can be useful. - doesn't get everywhere and the little sheets of paper are pricey.

A little 1/6 sheet finishing sander with an inch wide chunk of dense foam between the sandpaper and plate. - plan on chewing through a lot of paper.

A teenager desperate for money and some 220 sandpaper wrapped around a foam block.

i wrapped a teenager around a foam block once. ;-]

seriously? Pam Wedd had a real good series of articles on sanding, varnishing, painting. they were in the Journal although I can't recall the nos.

One of the hints related to sandpapaer itself---- fasten a hacksaw blade to something solid, like the door frame. You can slide the sandpaper under and tear it off just like waxed paper. Then get it to where you can fold it in thirds. Thirds keeps the sandpaper grit from touching itself and stays sharp when folded that way. then go cross grain eek. but with fine 220 320 400 whatever you can live with. then "heavenly" varnish.
I think she sanded the planking with somthing more fine than was used on the ribs. If memory serves.
I use a fine grit sanding sponge. It follows the contours well and hits the sides of the ribs as well. Buy a few of them.
No, it leaves small pieces of steel behind, that turn into dark rust spots.

In addition to all the other methods, I added a small block about 5/8w by 2" long to the end of a triangular shaped viabrating sander. I then cut adhesive backed sandpaper to fit. With it i can get between the ribs and work with the grain, but it's slow. :(

I Got It!!!

Take two strips of Contact Paper and apply them to each other one on top of the other. Cut the strip to match the contour of the ribs edge and maybe another 1 1/2" wide. Place the laminated contact paper strip sticky side down on exposed planking that lies between two ribs. This protects the planking from cross grain scratches as you sand the rib edge. You can use the method to cover the edges of the ribs if you have serious plank sanding to do too. The contact paper will lift right back up and you can re-use it many times. It stays pretty sticky if you vaccumed the inside of the hull well before sanding. I use this stuff for anything I want to protect from some thing I'm doing to a thing next to the thing..... Well, you know what I mean. For those that don't know or remember what Contact Paper is, it's a plastic sheeting that comes in a roll. The back has an adhesive film and it's covered by a waxy paper that you pull off before applying to whatever surface you want to cover. They come in many fashionable patterns. For a situation like this, I think we'd all agree that a wood grain should be chosen.