How about sand blasting?

Or a corded drill with a large wire wheel?

 

You probably mean clapboard siding. You didn't say if it is painted, stained or unfinished. There are numerous products available to strip old finishes as well as some to wash and brighten old wood. They are often sold as deck products but can be used on wood siding, too. Depending on your conditions, you may be able to do the job with a garden sprayer, scrub brush and a garden hose. This supplier has a number of products as well as good info on prep work and sealers.
http://www.opwdecks.com/

I use a normal random orbit sander for a lot of paint prep work. On clapboards, they don't get into the corner between clapboards and you have to control the pad from hitting and bouncing. The unreachable areas can often be addressed with a properly sharpened scraper. You can't put much pressure on the sander, let the paper do the job.

Here are a couple tools, one inexpensive, one not, that are used for removing paint on clapboards and other prep work.

Porter-Cable 7403 8 Amp Abrasive Paint Remover - Amazon.com

Amazon.com: Wagner Power Products 513040 PaintEater: Home Improvement

 

Thanks

Thanks guys....great suggestions and it is clapboard.

 

Once your finished sanding. Remember Mr Miyagi says paint the house side to side and switch hands for every board.:thumbsup::laughing:

 

Before you begin determine whether you are dealing with an old finish that contains lead. If it was painted any time before the mid 1970's you may have a problem. Sanding will spread the lead containing dust all over the place. You would need to protect yourself by using respirators and consider any neighbors that live in the area.

I've seen lead detection kits in some hardware stores. Some big boxes may have them too.

 

Check for lead (if it was ever painted), then try a pressure washer. That should blast off all of the "grunge" that will clog sandpaper like crazy. Then sand where neded. Deck stain or paint to suit.