I had a problem recently with masking fluid that wouldn't come off my paper.
I've been producing a series of Jacobean flower inspired paintings. Part of my process is to mask off certain parts of the painting to help keep the images light.
We've had some extremely hot days so far in our summer down here in Oz, and I think it affected the fluid. I use Winsor & Newton, normally without a problem, but this time it went sticky, changed colour and wouldnt rub off the watercolour paper. I was using a heavy 300gsm rough paper, (Saunders I think).
After some experimenting, I successfully lifted the masking fluid from the paper by using a plastic eraser (Staedtler Mars Plastic).
I cant guarantee this method will work for everyone, but I thought it worth sharing...
The trick is to use very light gentle strokes, to created lots of very small rubbings no more than 2mm wide. This slowly lifted the masking fluid without ripping or marking the paper.
When I tried heavy pressure, it didn't lift the masking fluid, in fact it applied a thin coat of plastic eraser on top of the fluid and left a grey residue.