The best masking fluid

I'm in kind of a tough spot. I'm working on the Black Widow and I made some errors in judgement. I glued the fuselage prior to painting but in some cases I had to like the cockpit however, it's so detailed that I had to detail it before it was inserted into the fuselage. I have cannon protruding from the upper turret and well as the side cannons, same situation as the cockpit. I can't use masking tape since the parts are too small. Any suggestions would be much appreciated.

Gwaredd

Hi, and welcome. This would have been better in the Modelling section, under the Painting questions, but not to worry.
Probably the easiest to find in the UK is Humbrol Maskol, which is also available in the USA. Practice with it first, as all masking fluids have a tendency to 'shrink back' slightly from the outer edges. (slightly diluted PVA can also be used, but may be more difficult to remove.)
However, if this is for masking the guns and other small parts, it's possibly not suitable, as there may not be enough area to obtain the surface tension, and completely mask the part in question. Also, removing the dried fluid could damage delicate parts. (it's normally best to find a way to leave off all small parts until after painting, whenever and wherever possible).
But, all is not lost! From your brief description, I'm assuming it's the small items such as the guns you want to mask? If so, then wrap small pieces of kitchen foil, or 'cling film' around them, pressing this into place loosely, so as not to damage the part(s), but tight enough to prevent them being blown off by the airbrush, or knocked off by a paint brush, when handling.
Hope this helps.