Good point Pat! But the word 'that' could have two roles, hence the ambiguity.
Firstly the 'that' could be relative to the word 'gloss' in the previous posting. So, to the question 'What is gloss paint?', I could reply 'That is the top-coat in the painting process.
Secondly, for me to have missed a word out, (a frequent aberration on my part), then the clauses would be
'I think' = Main Clause
'that (it) is the top coat of paint' would be the Subordinate Clause.('it' would have been better)
So, Pat, to have been accurate and precise, I should have written "I think that gloss is the top coat of paint".