I see that there is not a lot of consolidated knowledge for these mediums online. Besides TDH, it looks like mini-modelers are the main people using them, though they are aimed at watercolor painters. I have had the opportunity to use four of the most commonly recommended ones, though I know there are others, as well as a lot of interesting tools people may use. I was hoping that some of the ultra-experienced here could add to this so that any newcomers could have more luck than I did when starting using these.
The ones below are what I have used so far and the pros and cons I have identified:
1. Toothpaste was my first "mask". I have seen this recommended a lot and have used it several times in a pinch. I would not recommend it, though it leaves your hard work minty-fresh. It is messy and generally sort of sticky, which can make it a pain to remove. The paint does not dry on top of it, and can create smudges when removing. I suppose most of these may vary with brand used. I used UltraBrite.
Pros: Cheap, easily obtainable, smells nice, noticeable under paint, large quantity.
Cons: Sticky, hard to remove, not easily spreadable or shapeable, doesn't let paint dry, and overall hard to work with.
Overall: Not recommended...
2. MicroScale MicroMask. I saw this recommended by a member on here along with a couple others and decided to give it a shot...twice. Mistake. I will say that I like how it applies to surfaces and it able to spread. It comes in a deep blue hue, which dries to look nice and is super easy to see. It was great up until being painted over. Even when applying what I considered a lot, it dried to be so thin it was next to unnoticeable even under a thin enamel layer. There is no easy way to remove it. Seriously, it dries onto your parts like a glue and you will likely damage your paint trying to remove it. After reveling in ruining a layer of paint I found online that other modelers use it to mask over model car and aircraft cockpit glasses. It comes in a minute 1 fl. oz. bottle.
Pros: Cheap, Shapeable, Easy-To-See, Easy-To-Obtain.
Cons: Sticky, Hard-to-remove, Dyes Fingers.
Overall: Don't waste your money.
3. Humbrol Maskol. I finally submitted to the will of TDH and bought a bottle of this. I am U.S.-based and my only qualm with this is the price per bottle, which is not particularly cheap. U.K. or otherwise might just stick to this one. I don't recall having a single problem with it for the duration that I used it, and I used 90% of the bottle. It is quite the interesting texture and you are able to rub it between your fingers into little latex boogeys. It applies great, dries fast, and comes off pretty easily with X-acto or tape. It leaves nice clean edges to your shapes. I was disappointed to find that the remaining 10% had dried up in the bottle after having not being used for only a month. The bottle is only 28ml, which goes a long way I guess, but is actually less than 1 fl. oz. U.S. Amazon does not offer it, though you can get it on eBay for a reasonable price.
Pros: Dries quickly, Colored, Easy To See, Easy To Apply, Easy-To-Remove, Non-Damaging, Clean Lines, Easy To Clean Up.
Cons: Expensive (U.S.), Small Amount.
Overall: Maybe my personal favorite, but if you're a penny pincher you might hold off.
4. Winsor & Newton: This is my most recent one and it is very similar to Humbrol Maskol, though noticeably runnier. I have seen it recommended about as often as Humbrol on here too. It is yellowy-clear out of the bottle, which makes it harder to see than the others, though that is quickly remedied with food coloring of your choice. It spreads and applies well, dries quickly, and leaves nice lines when done. Again, very similar to the Humbrol, though it's viscosity may hinder you from using your usual applicator of choice. I had a couple nice surprises when ordering this. It is Amazon Prime eligible, which is nice. I received 3 bottles of it though I only ordered 1 - not a pro, I just wanted to brag. That experience made me want to say it was cheap, though it is similar to Humbrol in the price aspect as well... I have seen people comment on the smelliness of it, though it is no worse than the Humbrol. The best part though is that it comes in a 75ml or 2.5 fl. oz. bottle. 2.5 times the bang for your buck than the other ones - so I conclude that it is cheap.
Pros: Dries Quickly, Cheap, Easy To Apply, Easy to Remove, Non-Damaging, Clean Lines, Large Quantity
Cons: Needs Dyed, Pretty Runny.
Overall: Probably the best bang for your buck, though maybe just one degree less easy to use than the Humbrol. Toss-up.
5. Miskit by Grumbacher
per TDH user Deriak: It applies fairly easy but tends to be a bit runny if too much is applied like for larger areas if there is any contour like the kidney plate and the back plate. I am sure I will run into the same issue with the COD plate also. It dries nicely. I can usually just run my finger nail across the area just to get it going. Then i can just rub it off with my finger. Smaller areas are a bit difficult to see and I have to remember where I put it at. I usually refer to my stencils to see where I was putting it. It come in a peach/pink color, but drys almost clear. I have not tried to use any dye. I may try to do that.
Those are just the 4 I have used so far, but I know there are others available. I know of at least one prop maker who uses yellow mustard to mask... YUCK.
As for tools, I would like to see what the community has to say. I generally use a mechanical BIC Pencil #2 with .7mm lead to apply my masking details. I just press out a lot of lead and use it like a pencil obviously. The masking fluids do not allow for the lead to write on my surfaces, though I noticed that it would transfer when using toothpaste. Other tools I have used are toothpicks, unbent paperclips, and brushes. I do not like to use brushes because the masking really screws them up and none of them really come out of the bristles.
Hopefully this comes in handy for someone along the line. I'd be interested to see what other people use for masks and applicators, as I am by no means an expert painter! Happy hunting.