The best way to prevent fog is not with chemicals or any other junk. Its called a bandana. I use a sweetband personally (wilson tennis to be exact), but its up to you (hats work too). Ever wonder why the pros in your paintball magazine are wearing bandanas or hats with there mask? It's not to look cool, thats for sure. Fog is maily caused by sweet (warm) from you face drooling under/around the foam of the mask and causing it to get all foggy. Sometimes even with a hat or whatever, the sweet that develops from the part of your face actually within the foam, is still enough to cause it to fog. This will happen a lot quicker without something stopping the extra sweet though. If fog is still a problem using an bandana, thats were anti-fog stuff and thermal lenses come in. I have never felt the need for a thermal lense, or extra anti-fog.
As for having problems with paint stuck between the lense and mask frame; I've never had this problem because I have a V-force. Although they are not hugely overpriced, you may still want to look at them. I have there cheapest one (armour) and have no problems.