@chronos, this dime could have been buried or exposed to saltwater for a time. This has caused surface staining and uneveness to the surface. However, those striations look to me like they are resulting from this '87-P dime having been struck by a die that was starting to wear out (e.g. VLDS).
"If you climb a good tree, you get a push." -----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed." -----King Adz
Or riding in a cup holder in a car with soda spilling on the coin. Not an error. Just post strike damage. Note a pattern to the staining? In a cup hold many/few coins can be affected, or blocking the affect of the staining depending on where the coin is in the holder.