Scott,
You've got a good respirator in the 3M line. We've had a little experience with what you're smelling, except what we experienced was something akin to an ammonia-type odor, and we experienced it with the MSA brand. The manufacturer assured us there was no problem, and was simply a reaction of the cartridge with moisture in the air. We noticed this on a daily basis during a 2 year PCB building decontamination project. No ill effects were noted by any of the team members involved. If you are bothered by the smell, or if it gets worse, I would recommend getting some newer cartridges. Could be that the reaction is more prevalent in older products.
The other problem may be that you are actually smelling the vapors in the thinner or the paint you are using. First, make sure the cartridges are seated well in the cartridge mounts, and that the rubber seat rings are properly installed. Run your finger around the ring and feel for any grit or other particulate matter that could be interfering with the seal. Next, check both the inhalation and exhalation valves to ensure they are properly seated and functioning on their mounts. The valves should be very flexible, and free of any tears or cracks.
Now put the thing on. Attach the neck strap first, then bring the respirator up and over your face and put the head strap in place. Make sure you have a snug fit. If you have a beard or other facial hair that may interfere with the face-to-facepiece seal, you will probably get a little leakage in the mask and odors may be discernible. Facial hair will diminish the effectiveness of a respirator.
OK, now that you have the respirator fit to your face like you want it, cover the cartridges with your hands, or a small plastic bag in each hand if your hands are too small, and breathe in. The respirator should collapse on your face. Hold your breath for 10 seconds. If you have a good fit, the respirator will remain collapsed. If not, you have a leak. This is called a negative pressure fit check. Next, cover the exhalation valve with your hand and breathe out slightly, but not enough to break the seal. The respirator should expand on your face. If air escapes, then you have a leak. This is called the positive pressure fit check. If you notice a leak while performing either test, readjust the respirator, and recheck. While it sounds involved, it really isn't, and should be done every time you put the respirator on. Otherwise, you spent $20 for a false sense of security.
If you still can't get a good fit, then you may have the wrong mask size. Masks come in S, M, and L.
Once you get it all together, try this: Get some fingernail polish remover and dip a q-tip in it. With your eyes closed and breathing through your nose, run the q-tip around the perimeter of the facepiece, paying special attention to the facepiece-to-face seal area. This is called a qualitative fit check. If you can detect the smell of dilute acetone, you have a leak. Re-fit and try again. Typically, if you have a good negative/positive fit check, you'll also have a good qualitative fit check. If you can smell the acetone and have a good fit check, the cartridges may need to be replaced. Normally, this check is done with n-amyl acetate, which can be found at some drug stores. I recommend trying it with nail polish remover because it's readily available, the acetone is dilute, and n-amyl acetate (banana oil) may not be available everywhere.
Lastly, only one pair of prefilters go on at a time. The prefilter is used to trap particulates only.
If none of the above work for you, let us know, and we'll try something else.
Hope this helps.
Gip Winecoff