I had great luck when painting around my 180g freshwater tank by roughly sealing off the tank with plastic sheeting and tape, with a big, pond air pump pumping outside air (or air from an area not being painted) into the tank itself!
This creates a bubble of positive air pressure inside the plastic sheeting, bleeding air out through every crack, preventing pain fumed house air from contacting tank water!
— the same principles can be applied in a reef tank! You may just be able to plumb your skimmer to outside air to achieve the same effect, but a high volume air pump may be the safest option! (And, it would feed the skimmer fresh outside air from the positive pressure bubble!)