In a word, it's impossible, especially in hot, humid climates.
We hang a towel on the chest strap of our SCBA harness.
When we need to wipe the faceplate, we pull an arm out of the suit, grab the towel, wipe, re-hang the towel, then put the arm back into the suit arm/glove.
Once someone invents an air-conditioned Level A suit, maybe we'll eliminate the fogging problem.
I live and work in Santa Barbara, CA. It's not uncommon to have the Level A suits fog up... But the good news is that we found a way to deal with it and I'll share it here for you.
Needed Equipment: Plastic OSHA compliant hard hat with a brim + large feminine hygiene pads + duct tape.
We always have our guys wear a hard hat when donning Level A, you never know, wouldn't want anyone to smack there head on something. So we figured out that if you simply tape a large hygiene pad onto the brim, you just have to take a flat hand against the polycarbonate face shield on the Level A and wipe away any condensation. It works better than looking through a fog cloud... Hope this helps...
On another note, I am on a hazmat mission that requires input from all of you. Do you have toxmedics on your team? Have you had access to the Advanced Hazmat Life Support training? Do you carry antidotes for you team members in case something goes wrong?
Please consider visiting a new FFN site called Hazmat WMD Responders. This is where I am placing this information. Thank you for your consideration. Mike from Santa Barbara
I have used a light coating of soap on the inside of the face shield before. It works for a while but needs to be wiped off and re applied. Also used the towel taped to the helmet brim trick as well. Stay Safe!