I'm not sure exactly what you're trying to record, and where. Also, I don't know if this is a "one-time" fix, or if this is something you need to continually fix on a regular basis.
How adverse are your recording conditions? If you're trying to follow people around like a reporter would (it doesn't sound like you are) that's one thing. I have little experience in that type of recording.
But, if you're working in a controlled environment (you mention 'portable studio' so it sounds like you are) I might make some suggestions.
Unless you only need a one time 'fix', attacking the source of the problem rather than trying to "fix it in the mix" will pay off in the long run. Try moving the microphone a little further away from the voice- about 10-12" back. Use your screen filter placed equal distance between the mic and the voice (5-6").
If you're making digital recordings (as opposed to tape recordings) don't hit the 'red' zone on the recorder. Keep recording levels at least 3dB under that considered to be the 0 db level. If you're already doing that go further - try recording another 3 dB under what you consider to be 3 dB down (6 dB total). Try turning the padding down on the preamp, if it's available and adjustable.
Although it really does sound like more of a mic problem there may be a compatability issue between the mic and recorder. Experiment with different mics. There ARE some bad mics out there.
Finally, if you really can't figure out the source of the problem, or if you just have one recording to fix you CAN fix it digitally afterwards. Ten years ago I used to use a program called Popfix. I don't know what programs you have availble. Modern recording software offers more built-in "fixes" these days.
Good luck!