While there are several auto filters in PS to produce painting effects they do not allow you full creative control.

Here are three methods, the first one fairly automatic once you have selected a couple of settings, the other two allow you full control over brush size, pressure(with a suitable graphics tablet) etc.

Method 1 for a watercolour effect

1 : open RGB 8bit image
2 : make dupe background copy > filter. artistic> cutout; to start use default settings as you can always go back and alter them latter. > OK
Change blending mode to 'luminosity'
3 : make new dupe background and move to top layer > filter > artistic > dry brush > default settings > OK change blending mode to 'screen'
4 : make yet another dupe background > filter > artistic > paint daubs use default settings > OK. Take this layer to the top and change blending mode to 'lighten'.
5 : If the image looks too light or dark make an adjustment layer > levels and adjust to suit. if the textures and brush strokes need adjustment, activate layer 2, 3 or 4 and open the filter setting used in that layer and alter the settings. Each image will need different settings so experiment.
6 : save, I think I'd save on a cd with the layers which gives a very large file but does give you the option to go back and modify the image.

The result should look like this

Method 2 for an oil painting effect

Open an 8 bit RGB tiff

Select layer > pattern > select a suitable pattern, the natural textures work well; this image has used dirt at 95% size.
change blend mode to 'overlay'

add a new empty layer
Select the smudge tool. check All Layers button. Use a brush size to suit the area to be painted, use a brush from the oil brushes. Experiment with the opacity and strength controls. The background of this image is 25% strength and the flow 90% both at 100% opacity.

Paint over the image as you would if using real oil paints, short strokes following the contours, you'll find the colour will blend at high strengths.
To check your progress you can occasionally deactivate the background layer.

Duplicate this painted layer and ensure it's the top layer.
Select filters > stylize > emboss play with the controls and light direction to get the effect you want.
change blending mode to 'overlay'

Use an adjustment levels or curves layer to adjust the tonality

Save, flatten if you don't want to go back and modify the image. Again I think I'd save on a cd with the layers which gives a very large file but does give you the option to go back and modify the image.

Experiment with different patterns, brush sizes and shapes.

With this technique YOU have complete control over how the final image looks.

A couple of tips; the patterns that seem to work best are the 'stone collection' use them at about 60% size. check out the full brush list and use the oil paint ones and use them at about 70-80% strength and mode normal.

Method 3 for a more controled water colour effect

Open an 8 bit RGB tiff

Select layer > pattern > select a suitable pattern, the paper textures work well; this image has been used at 55% size.
change blend mode to 'overlay'

add a new empty layer
select the smudge tool. check All Layers button. Use a brush size to suit the area to be painted, check out the brush list, you'll find the water colour brushes towards the bottom of the list. Experiment with the opacity and strength controls. The background of this image is 25% strength and the flow 90% both at 100% opacity.

Paint over the image as you would if using real water colours, longish strokes following the contours to give the effect of a wash, you'll find the colour will blend at high strengths. Use smaller well loaded brushes for the detail work.
To check your progress you can occasionally deactivate the background layer.

Duplicate this painted layer and ensure it's the top layer.
select filters > artistic > water colour play with the controls to get the effect you want.
change blending mode to 'overlay' and an opacity of about 50% or less.

Use an adjustment levels or curves layer to adjust the tonality.

You may also wish to experiment with other filters such as diffused glow on another layer.

Save, flatten if you don't want to go back and modify the image. Again I think I'd save on a cd with the layers which gives a very large file but does give you the option to go back and modify the image.