Where should the adverb "poorly" go in this sentence?
- The primer and coating poorly react to...
- The primer and coating react poorly to...
Both forms are correct, however the second option sounds more fluent.
As described here, adverbs that describe the manner of something usually go at the end of a clause, unless they are not the most important part of the clause.
In your sentence, "The primer and coating react poorly...", the emphasis is on the adverb "poorly".
This is because the point of this sentence is not to tell us that the primer and coating react, but rather the extent to which they do. Thus, it is more approproate for the adverb to be at the end.
In your first sentence: The primer and coating poorly react to... This use suggests that you are expecting a reaction, but your result is poor. Which means there is a reaction, but just barely. An example would be if the coating was supposed to be non-reflective and ends up reflecting light. In that case, it poorly reacted to the test condition.
In your second sentence: The primer and coating react poorly to... This suggests that while testing, there was a condition that damaged the primer and/or coating. An example of that would be a weak acid that partially dissolved the coating. In that case, the coating reacted poorly to the acid by dissolving partly.
Standard use would be the second phrasing.