That's good advice from Andy and Marc, and I'd like to add some recommendations:
Model Master acrylics seem to have more trouble with adhesion than most other respectable brands. But ALL acrylics benefit to some degree from priming. Personally, I use a Tamiya light gray acrylic as primer, because Tamiya acrylics adhere very well when properly applied.
Properly applied defined: To a sterile surface. No, I don't mean use an autoclave! But the model surface must be free of all contaminants. Regardless of what you use to wash the model, the final rinse should be with distilled or deionized water—this can be just a wipe with a staturated, lint free cloth or paper towel. An alcohol rinse/wipe also works, although I would recommend using ethyl/denatured alcohol rather than isopropyl, as they evaporate faster and have a greater affinity for water. I also prefer washing in a Simple Green solution or using a surfactant cleaner. The latter, allowed to evaporate fully, sometimes helps adhesion. However, surfactant cleaners sometimes vary in composition over time. Windex (with ammonia) is a fairly reliable brand and evaporates completely.
Finally, before applying your masking, make sure that the underlying paint is fully cured. This is critical. Acrylics dry rapidly, but cure slowly, especially with regard to surface bond. This is just as true for Future (PPFTWFutureS) as for any other acylic coating you are likely to use on a scale model. To insure full cure, wait at least a full 24 hours at room temperature and not more than 50% relative humidity. Force curing with mild heat and circulating air can reduce this time to an hour or two, and is especially useful in cooler, damper conditions.