What is arepa flour?
From  Cook's Info :

Arepa Flour is a refined, pre-cooked corn flour (&quot;harina de maiz refinada, precocida&quot;.) It is not the same as the masa harina that is used in Mexico; for Arepa Flour, large-kerneled corn is used that has large, starchy endosperms, making for a starchier flour than that which is used for tortillas.
There are white and yellow varieties of the flour, because the corn used may be yellow or white.

Also:

For commercial makes the corn is hulled dry. The hull is first removed from the seed, as well as the germ. Then the kernels are cooked, ground, dried and packaged.
Arepa Flour is sometimes referred to as a &quot;pregelatinized&quot; flour because the grain has been cooked first.

I'm thinking that masa harina would not be a suitable substitute for making Arepas. Another thing I've found is that corn flour is usually finer than corn meal, even if the corn meal is ground 'fine'.