I'm not an expert at montage, but I have had this problem before too. What i did:
1. made a layer that was a solid color from the background my pattered paper.
2. placed the layer in 1. between the photo and paper
3. added a layer mask to it with the 'hide all option'
4. painted on the layer mask to reveal the solid color in areas where the pattern was showing through.

It worked for me, but I really find montage is more trial and error than anything. hope it helps!

anna

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