Hi Dan,
Your Spencer looks like it would be a good candidate for rust cleaning via electrolysis. It is a non-abrasive way to remove rust. All you need is an old phone charger, two pieces of steel, some copper wire and a plastic tank. I use one of those "under the bed" storage units that they sell at Home Depot. You can find information on other forums and the Internet on how to set up an electrolysis tank for cleaning rusty firearms. Being new to the forum, I don't now if forum rules allow us to post links to other forums. If that's allowed, I can post a link to a tutorial on another forum.
I've used the electrolysis method to clean an old Rolling block and a M1842 musket. The surface rust and grime lift away. When done, all you need to do is gently scrub the metal with 0000 steel wool/WD-40 to remove the loose grit/grime/rust then oil the parts to prevent further rust. You are left with a dull grey patina and absolutely no signs of abrasives or scratch marks.