How to fix wavy surface

[Ofnuts]

The wavy aspect is likely a moiré pattern (as are the vertical darker stripes). This happens because the spacial frequency of the fabric is close to the spacial frequency of the sensor (in other works, threads in the fabric almost match pixels (or a multiple) in the sensor).  

Normally best avoided at shoot time:

• by shooting closer or farther to change the spatial frequency of the fabric on the sensor
  
• by blurring the picture a bit (focus blur, or the oold nylon-hose-over-the-lens trick)
  
• by taking two to four picture with slighly different positions (which will display the moire pattern a lot), then aligning them and blending them with a median filter, average filter, or "lighten/darkenen" only blend mode.
  
• if you shoot raw, you demosaiciing algorithm can have tweaks to suppress moiré (or you can have the choice between several algorithms, more or less prone to moiré).
  

In Gimp, a solution to mitigate the problem.

• Filters ➤ Enhance ➤ Wavelet decompose
  
• Inspection of the result shows that the problem is mostly in the Scale 6 and Scale 7 layers:
  

   

• In these two layers, sample the color in that area (with a large sample average radius), and use a soft brush to repaint the problem areas to make them more uniform.