Masking is a technique used to cover portions of your model to keep unwanted paint off of certain areas. This becomes extremely important when using an airbrush, which as I mentioned way back on Day 1, is something I don’t use! So my discussion of masking will all be second-hand.
There are numerous ways to mask your model depending on your preferences and budget. A popular choice is masking tape. The same stuff you buy at the dollar store can work in a pinch, or you can buy tape specifically designed for models (Tamyia’s Masking Tape is a well-known brand name). Tape is particularly useful when you need distinct, sharp lines.
The drawback of tape, like any adhesive, is the risk of damaging any previously-painted areas underneath it. Masking tape works best when you can apply it to areas where you haven’t painted yet and want to keep paint off of, but your mileage may vary. Plus it takes a lot of time to tear off and slap on all those bits of tape.
Another option is masking putty. Again, you can get modelling-specific products like PK Pro’s Masking Putty, but I’ve also seen people use Silly Putty, or the sticky tack putty that you use to hang posters on the wall. This one has the advantage of flexibility, as you can easily shape it to your area to mask, as well as it’s quick to apply and remove.
A third, crude but sometimes effective option is covering it with layers of thin solid material like cling wrap or even paper towel. This usually only works for larger models and is certainly not as precise as the other options on this list, but it’s helpful for covering very large areas where using tape or putty would be awkward or just overly time-consuming.