Hi,
You have 2 options here. Depending on the complexity of the area to varnish:
A- the quick and easy choice:
Import your picture in InDesign
Create a new spot color in your swatch palette https://helpx.adobe.com/ie/indesign/using/swatches.html
The colour that you choose (bright pink for example) will not print in that colour, it is just to help the printer seperate that information from the rest of the picture and print the varnish separately.
Use the pen tool to draw the shape you need over the picture and fill it with the spot colour
B- If you need a very accurate and complex shape for the varnish
in Photoshop
Create a new spot chanel https://helpx.adobe.com/ie/photoshop/using/printing-spot-colors.html
On a new layer, paint the area of varnish, again the colour you choose does not matter as it is only for area coverage information, not using the exact volour you picked.
Save the file as a PhotoShop document
Import the psd in InDesign, the spot channel added should also appear in the swatch panel as a new colour
Contact the printer to check what pdf settings is best for them
export and send pdf
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