Masking fluid is a great product for watercolourists. It allows us to mask off areas that we want to keep light without having to paint around them and, because it is removable, masked areas can be modified later. This is particularly helpful for very small areas, such as the highlights in eyes or on a drop of water for example. All the leading manufacturers produce it. It is available as clear or coloured liquid in small bottles (generally up to 75ml) and is inexpensive. It can be applied with a range of tools; cocktail sticks, draughtsman’s rule, paper, card, string etc. but using a brush is ultimately a death sentence for the brush (if you condition the brush by treating it with soap before using it for masking, it may last a little longer as the soap helps shed the masking fluid after use). You can sometimes buy it with a built in applicator.
You are only likely to use tiny amounts at a time so decant a small amount into a smaller container and work from that to avoid the whole jar drying up - I use contact lens pots or bottle caps.
Only ever apply to a completely dry surface and let it dry before painting over it.
Remove (by gently rubbing with a your finger) when the surface is dry - if any moisture remains it can cause the paper to tear.
Try experimenting with different application methods.