I was reading a book which used the word render as a noun. In its context, it seemed to be part of a masonry wall. A quick trip to the OED showed that it is the plaster covering on a brick or stone wall. Amongst the two different nouns render, the first being a rare nomen agentis for the verb rend and the other a deverbal noun meaning a payment in kind. My question is why the term render that I looked up was included with this latter noun render, rather than being a third entry? (It's cited from the late 19th century.)

I'd say because it is from the same verb form, since what the render does is transform the appearance of the wall by rendering it into something else. Under the verb render, one of the definitions is about coating a wall with plaster.

I'll admit to not being extremely familiar with how the OED is organized, but it looks to me as though in this case they have the five different noun uses carefully listed and categorized "correctly."

The OED, like most modern dictionaries, jas separate entries for different words. This means that render in its various meanings, i.e., 'to give in payment', 'to melt fat', and 'to plaster', is the same word from Old French from Latin. I was just wondering how it went from pay to fat to plaster.