When painting Thagrosh, masking became very important because I had to keep paint off of all the crystals when priming and airbrushing. I could have glued the crystals in after priming, but then the green stuff has to be sculpted in a way that allows me to remove the crystals. It wouldn't look as good.
Some notes on Masking TapeMasking tape differs from other tapes in that it has a "paper" look and feel. Cheap masking tape differs from what you'd get at a hardware or paint store in that it's basically just "paper-like" tape unsuitable for "masking".
Good tape will usually be rated in different strengths, stickiness or number of days you can leave it on a surface without leaving behind residue. Stronger tape will be more sticky and not rated for many days.
For our purposes, get the weaker tape. It'll be easier to remove and won't leave behind messy residue on the surface of your model.
The decision you may be faced with is how precisely you want to cover the seem between the two objects. What I mean is in the photo above, some of the crystal is left uncovered by the tape. The other option would have been to cover all the crystal and some of the green stuff. If you have some Gesso, you can always paint on some primer over anything you cover that still needs priming afterward.
I left the masks on since the next step was airbrushing. Basically, whenever using a technique that indiscriminately gets paint everywhere, use a mask. The exception is painting something like blood splatters, where you'd want blood to get everywhere.
You'll notice on the right wing, one of the pieces of tape fell off. Unless you're using really sticky tape, this can happen with such small sections. It sucks, but at least paint can be scraped off crystal.
One is to finish painting the first area, then mask it off with tape. The issue here is tape could leave residue or even tear paint off of the surface you've just airbrushed.
The other option is to use a piece of card stock or paper and hold it in place to protect the rest of the model as you work.
In some areas though, I painted over the crystal. This created a nice effect as it applied the colour of the skin to the shape of the crystal.