Can a liquid mask cause clear plastic to fog?

If you want to try a different liquid mask, I've used Mask It Easy from Micro Mark successfully on my canopies for a few years now.  I prefer to cover large areas with tape or foil, then use the liquid mask on the open areas.  I apply it with a brush so I get it exactly where I want it.  It applies a little thick so its easy to 'roll' right up to the edge.

It works just fine with Future dipped canopies as well.  BTW, I've found Goo Gone works GREAT to remove any tape or foil residue left over from masking and leaves canopies sparkling.

Good luck.

One the model has had a sealer coat, the plastic portion of the model should be fine.  However, I have had several problems with using a liquid mask, I learned the hard way :-(   My prefence has been to use Mr. Top Coat between coats, but others may have a different prefence.  I have also expermented with using a liquid mask for a camo pattern with very poor results.  I would suggest that you use a stick with either a masking tape or silly putty (the kind at a Toystore).  As an additional suggestion, from pass experiments with resin, you may end up with leaving a build-upfrom the liquid mask on the model, very noticable after the top coat has been applied.  BTW I have used several brands of liquid mask and resulst do vary.

In addition to what Karl said, prolonged exposure to ammonia can etch styrene. The liquid masks that contain ammonia hold that chemical in close proximity to the plastic as long as they are in contact, because all of the ammonia never escapes the cured mask.

And Karls advice also applies to any acrylic paint  you may have used on the model. 

If you've dipped your canopies in Future, you can can't use an amonia based masking agent as it will eat into the Future.

Karl