How to thicken hair in Photoshop

Open your image and select the Healing tool. 

Remove any obvious stray hairs around the edge of the hair line and facial hair.

Next, go to Filter > Liquify and use short strokes to give the main hair line a little more volume. Specifically for this image, I also neatened up the back of the hair line a little too. You can see the results in the next step.

This is the result of the previous step, with added volume to the hair and a neater back.

Still with the Liquify tool, neaten the shape of the beard a bit – here bringing in some of the hairs on the chin. Be careful – if you go too far, the edges of the beard will look too perfect and it won’t look realistic.

Now, lets go and start filling in some of those gaps in the hair.

First, create a new blank layer and set its blending mode to Darken. 

Then choose the Clone tool and set its blending mode to Darken too.

You also need to make sure your Clone Tool is set to Sample Current and Below.

This ‘Darken Clone’ tool is great for filling in areas where you have an almost identical source point to clone from.

Hold down Alt and click with your pen or mouse to set your source point and then brush in any areas you need.

For some of the more difficult areas, use a ‘Dodge and Burn’ layer. Click Cmd/Ctrl + Shift + N to bring up the New Layer dialog box. Set the blending mode to Soft Light and check Fill with Soft-Light-neutral color (50% gray). Press OK.

Select the Burn tool and set its Range to Midtones and its Exposure to about 20%. Then burn in some of the gaps in the hair.

This will be especially good for the sides of the hair where the hairs are shaved and there is little in the way of detail.

Next we’re going to add more detail to the hair. First change the Burn tool’s Exposure to 40%, and match the brush size to the size of the hairs on the beard.

Now use the Burn tool to bring in more hair detail. You can also lighten some areas of the hair if required.

For those sections that need just a little extra boost, lets use Frequency Separation. First press Cmd + Alt + Shift + E to create a new layer above the others with a flattened version of all of your visible layers.

In the Layers panel, duplicate your image twice and rename these to High Frequency and Low Frequency in the order shown.

Still in the Layers panel, hide the High Frequency layer by clicking the eye icon and select the Low Frequency layer.

Go to Filter > Blur > Gaussian Blur. You want to blur the image until the sharp detail has gone, usually about 4px. This is the layer that holds our colour information.

Select the High Frequency layer in the Layers panel and turn on its visibility again. Go to Image > Apply Image… and copy the settings shown in the dialog box above.

Once Done press OK. This layer holds our texture information.

Right now your High Frequency layer will be grey. Change the layer’s blending mode to Linear Light and your image should return to its normal state.

Now we’ve separated our image by colour and texture, we can begin our main edit.

Create a new blank layer between the High Frequency and Low Frequency layers. This layer is where we will non-destructively add in some hair tones to the skin.

Select the Clone tool (S). Go to the tool submenu at the top of your screen and set the sample area to Current & Below.

Choose a soft-edged brush and set the Opacity of your brush to somewhere between 10-20%. Don’t forget to switch your Clone tool’s blending mode back to Normal.

Hold down Alt and click with your pen or mouse to select your sample area and start blending and correcting the local areas that need attention.

And you’re done.