This is SO dependent on each of the documents, and your purpose. For instance, say the primary source is a papyrus fragment that says, "All Gugu hate war and flee from battle." You have no idea how to evaluate it without context – did a Gugu write it? Did an enemy of the Gugu write it? A disgusted ally? An admiring pacifist? A historian like Herodotus who is definitely secondary but we don't know to what sources so we treat him as primary, with our fingers sort-of crossed?

The answers to those questions are all going to be found in secondary sources, and I think that without them the primary source is nigh worthless.

BUT secondary sources are set in their own time and place, and may not think to ask and answer questions you now have – so you want to go back to the "primary" with that question.

Example: there is a wonderful mantra for Christians in Luke 15:31. "Son, you have always been with me, and all that is mine is yours." A gendered question might be, "Is this truly applied only to males?" And the answer is No – the Greek in the original for "son" is teknon, a neuter noun. So child is a better translation.
But further examination of the primary would show that it occurs in the middle of a parable about two boy-children, so (in that context) "son" is reasonable.
And without secondary sources, who would know about these nuances.

I was going to say athletes foot also. When in doubt, make an appointment to see the doctor.