Weaver-

I'd second what Dave Boyer said and add when I was doing a lot of stainless 316L sanitary dairy work the business I was working for used what we called either "grey" or "white" wheels with the white wheel having a finer finish.  They were 6" buffing wheel about a 1" thick a lot of their welder would use to polish out the weld completely altough I am of the opinion that in that weld- a single pass fusion tig fusion weld- done properly is perfectly flush to the tube surface and just scotch brighting or a stainless hand brush to wipe of any surface oxidation from the heat is sufficient and doesn't need polishing.  It was usually a fallback but-saver for less skilled welders on those jobs.  When you say boats, can we assume you are talking high end yachts and you are looking at safety railing on the sheer and bowsprits?  Maybe cabin railing? The higher the quality of the boats your doing the work for the higher the expectations on the finish.  There are tools available for putting the cope into the posts for top rail fitups you can locte online.  It's essentially a belt sander with a removable mandrel on the idler end of the belt that you replace with the correct diameter mandrel to match your pipe and vice for your post.  Clamp the post in the vice and a handle wth a cam allows you to push the post to the sanding belt and it copes out a machined fit for the top rail.  The weld for this fit can then be done with minimum heat input and resulting distortion.  If you google pipe notchers and hunt around you can find examples- Ercolina builds one I believe.  They are expensive but for that work, if you have a lot of it, might be worth looking at.  If you're as cheap as I am you'll probably figure out a way to build your own.  Also pre-coped "fishmouths" of different pipe sizes are available but I don't kow the source although you might get lucky looking through the Waner Co. catalouge, they do everything handrail related.  You use thes to save on having to produce a perfect coped fitting but it gives you an extra butt weld to do- gotta figure the savings to your costs welding vs fitting.  Handrail distortion has been known to make me throw tools across the shop and wake me in the middle of the night with hyperventilated anxiety, so if Dave could provide a better description of the jig to straighten top rail distortion he mentioned, I for one would be gratefull.