I clicked on your Banner Finds listed for June 2nd, 2013. The photo host is no longer supporting pictures of that find, but it must have been nice. I have several of the "Bustos". Looking at the detail on the reverse, the lettering for PLUS ULTRA is very
clear with little or no wear. The brooch on the armor on the King's shoulder is complete. There is still underlying luster. There doesn't appear to be any significant damage. Usually the rims have a dent or two. Considering these coins were hand made one at a time on a screw press, this coin is very well struck. It was EF+ to AU+ when lost. If you have tried to pick off any of the corrosion and there isn't any pitting uncovered, it would be worth your while to investigate possible restoration by cleaning.
Yes, the mantra "Don't clean coins" is heard throughout the land. The main thing to watch out for is abrasive cleaners that leave scratches on the surfaces. No collector likes that appearance on a coin. Sea salvagers use electrolysis. i have used nic-a-lene
with fair results on small areas, but you have to stay right with it. A small amount can be applied with the tip of a round toothpick. Wait 5 minutes or so, gently see if any blackened material leaves the surface. Dab with a Q-tip, repeat. A lot of dealers use coin dip, but they fail to complete the process. If you have bought any silver dollars, or like coins, at a coin show, and have them turn yellow-brown years later, the dealer didn't wash the coins after dipping them. After dipping, rubbing alcohol must be used to halt the action of the chemical in coin dip. Then you have to be very careful drying the coin as not to leave hairlines. It is just a matter of how far do you want to go. If you have more similar coins, in your case you mentioned five 8 R's, I think just leave them as you found them for an outstanding display. As "smokeythecat" has already explained paying to send them in to a third party grading service wouldn't get you any answers to their grade. You'd get "Genuine not gradable" or the dreaded "body bag". If I had them, I would have to try to clean at least one, and not the easiest, but the one that looked the most heavily encrusted. I would have to show you what I mean, and we are hundreds of miles apart, so ...
How about a scan of the whole find? Also, the when, where, how, etc. of that exciting find. Those are really interesting coins. They circulated in the United States as One Dollar until the coinage act of Feb. 2nd, 1857. I have read that they were used for several years after that even as late at the 1880's in the U.S. Southwest. Those Charles III, Charles IV, and Ferdinand VII, 8 R's have made huge price jumps in the last ten years. Even the heavily chopmarked coins are going for premium prices. I believe most of the 8 R's listed on ebay are overgraded. Since they are made with old technology, the grading of Spanish Colonial coins is subjective. Many coins that are actually high grade coins have weak or missing legends or details. There are several factors to consider, but it is obvious to me that this 1798 8 R's here was a like-new piece when lost. I don't know what a 1798 dollar would be worth in today's money. It might be fun to Google a money conversion program and find out.
Maybe Spring will finally come to North America and we can all get out and look for treasure soon !
