To be honest now I know what your wanting theres a completely different approach if you have a PC at the gaming table. You create a new layer and flood it with black then put the PC map under that black and show it to them - i.e. all black. Then with the eraser tool just start erasing the black from where they can see revealing the DMs map underneath. I.e. reveal what they can know about at the time.

You can use a VTT for stuff like this where one like MapTool and I think BattlegroundsRPG will reveal it automatically based on character vision capabilities etc. If you have a PC around when you play you will find using a VTT better in many respects. Although the V in vtt is for virtual table top - a lot of people use them in live play too to speed things up.