If you try to correct a WATERCOLOUR painted on "woodpulp" watercolour paper - generally the more economical brands - this can be pretty disastrous. Trying to wash or lift out a painted area, even gently, will often pull up the top surface of the paper; you can suddenly find that the paper breaks down and you begin to get little paper crumbs on the surface! Often adding more colour on top just creates more crumbs.
You will have more success, however, with a good quality RAG paper, since this is made from 100% cotton rather than processed wood fibre. You can attack this with great gusto, a toothbrush and plenty of water, and often take off lots of the paint without ruining the surface at all. I often worked on Arches paper, and if a painting became overworked, I would hold it over the bath, run the shower on it and scrub off the offending area! So the moral of the story is....either get your shapes and tones right on inexpensive papers....or work on better quality paper which will give you so much more flexibility.
An overworked OIL PAINTING can be saved during the process of painting by the simple process of scraping the paint off! Do this gently with the side of a palette knife. Otherwise, you can do a certain amount of overpainting if you place some newsprint paper down onto a thick sticky area, allow it to soak up some of the oil, then rub it gently without shifting the paint, it will lift some of the paint, then the painting can be left to dry somewhat before reworking. This is a technique called TONKING, some painters do it with thin muslin, or kitchen paper, or strong tissues like mansize Kleenex.
An overworked ACRYLIC is far less of an issue - the surface can be overpainted time and again without causing a problem for the underlying layers.
An overworked PASTEL is much trickier. Dealing with an overworked pastel depends on certain circumstances.
1. If you are working on pastel paper, and have not used any fixative, then instead of working on incessantly, trying to adjust shapes or alter colours, you have the option of BRUSHING OFF, using a stiff brush, this will remove a lot of the pastel on the surface, leaving a hint of colour on the paper, which can then easily be reworked. If you work on and on, over and over, you will lose the tooth (texture) of the paper, so brushing off is often your best option.
2. If you have worked with hard pastels, you can "correct" by SPRAYING WITH FIXATIVE, and then by working with a layer of SOFTER PASTELS over the top.
But....what if you have tried all of this, have brushed off, worked with both hard and soft pastels, have used fixative between layers, and it still is not right and needs further work but the tooth of the paper is totally gone...what then?
I came across this tip only recently, you might find it worth a try.
You need some plain BAKING SODA, or BICARBONATE OF SODA (depending on whether you live in the US or UK.
STEP 1: Here is my test sheet. On Fabriano Tiziano pastel paper, I put down several shades of reds and oranges, building up a good few layers until the tooth was virtually lost. You can see this from the peach colour at the top, there is no sign of red through it, meaning that there was no texture left. I sprayed with quite a lot of fixative, and then put the yellow on top.