Care should be taken with masking fluid (US: frisket). The temptation is to over use.

Advantages

• You can mask out large or complex areas - then apply a wash or washes over all
• You can have the mask going in one direction and the washes in another
• You can apply mask over the first wash (do wait until it"s dry!!) if you use a high quality paper (Arches works well)

Disadvantages

• Painting where a lot of mask has been used always have a certain look where there are many hard edges. Use mask only when it's really necessary and don't use all over the painting. Blend in the edges to avoid strong lines - wait until everything is dry then scrub gently with a scrubber brush (this has short bristles)
• You ruin your paint brushes really fast - I have recently been using Masque pen, blue masking fluid in a plastc bottle with a fine applicator attached - absolutely brilliant - here is a link to their site. click here. If however you wish to use a brush and masking fluid, rub your brush in a little hard soap or vaseline before dipping it into the masking fluid and paint for 30 seconds before washing. (Don't use your best sable brushes!).
• Don't use a hair drier to dry your mask. Heat will set the masking liquid and it will be impossible to remove. Don't leave your masking fluid on a painting for weeks it will stain the paper.
• Soft papers pull off the top layer when the mask is removed. Really frustrating as you are probably almost finished ! Always use a good quality paper (eg Arches). TIP: test the masking fluid on the side of the paper to see how it pulls off. Larger areas are harder to remove than smaller.