First lets start with making a stamped visible layer by selecting the top layer and simultaneously pressing SHIFT + COMMAND + OPTION + E on a Mac or SHIFT + CTRL + ALT + E on a PC. A new layer will be created on top of the layer stack. I usually rename this layer something like ‘Stamped’.
Select the ‘Stamped’ layer and click on ‘Filter’ in the menu then select ‘Noise’ and ‘Reduce Noise…’. Set the strength to 10 and set all other sliders to 0.
Click ‘OK’ to accept the settings and repeat the same step again to apply the effect twice to this layer. Depending on the resolution and the amount of details of your image you might just do this step once or a couple more to reduce the amount of details that will be sketched. Because I usually use this effect on digital paintings twice is usually enough. But if you prefer to try this effect on a photograph, you might want to try a couple of more times applying the reduce noise filter.
Now we need to desaturate this layer by clicking on ‘Image’ from the menu then select ‘Adjustments’ and choose ‘Desaturate’. This will turn the layer grayscale. Now click on ‘Filter’ and select ‘Stylize’ then choose ‘Find Edges’. Now copy this layer By clicking on ‘Layer’ from the menu and selecting ‘Duplicate Layer…’. The copy is now active. Change the blend mode of the copied layer to ‘Multiply’. From the menu select ‘Edit’ and choose ‘Transform’ and select ‘Scale’. In the top of the screen you can change the percentage of the scale transfer. Try a small increase like 101% or 102%. In this example I choose 101.5%.
You will see the sketch slowly taking shape. If you want even more sketchiness, you can make another copy of the ‘Stamped’ layer and change the blend mode to multiply and scaling it a little bigger then the previous layer to let’s say 105%. Dial the opacity of this layer down to 80%. Select the ‘Move’ tool and with the cursor keys nudge this down a couple of pixels (around 15 in this example). Now select ‘Filter’ from the menu then select ‘Blur’ and choose ‘Gaussian blur…’ Select a radius of around 0.8 to 1.0.
Now select all the sketch layers (‘Stamped’ and the copies you just made) and click on ‘Layer’ in the menu then select ‘Merge Layers’. Now rename the layer to ‘Sketch effect’ and change the blending mode to ‘Multiply’.
You can now add a black layer-mask and paint with a white paintbrush on the layer where you want the sketch effect to appear.
This is a quick and easy way to create a sketch effect that can be used as an extra special effect on your artwork. If you need a nice detailed sketch I would suggest using an action to create a more sophisticated sketch.
The action I use to do this is called Technicalart by Unicdesign ($8 at the moment of writing). Or you might want to try the newer version of this action. I haven’t tried this version myself yet, but I will soon and promise to write a short review for it on this website.