Editorial: Painting involves a sequential application of layers, each of which adheres satisfactorily only if the underlying layer has been properly applied.
Setup Claims
conditional (only if)
Each layer of paint → underlying layer adheres satisfactorily properly applied
analogy (X is, in this respect, like Y)
Education is like painting, in this sense that each new layer only sticks if the previous layers were properly applied.
The most important steps in painting are preparation of the surface and application of the primer coat.
Fill-in-the-Blank
It makes sense to suppose that ....
Evaluate
The language setting up the blank is signaling that we should be inserting a conclusion. The answer should be making a safely-worded inference that synthesizes the discussion so far.
The author just set up an Analogy between painting and education, so the blank should probably have the feel of Complete the Analogy.
Goal
Let's look for an answer like, the student's surface" and application of "the primer coat" to the student.
Since preparing the surface and then adding the primer coat are the first two steps of painting, we could surmise that the answer might say that the most important steps in education are the initial, foundational ones.