Usually we do our best to prepare surfaces so that epoxy sticks well. Whether it's a repair or modification, it's important to get a good bond.

But sometimes, you need for the epoxy not to stick - repairing a fin box and using a fin for a mold, or rebuilding screw threads in a foot strap insert.

Here's a good blog post on various methods of keeping epoxy from not sticking

https://www.epoxyworks.com/index.php/what-you-can-do-if-you-dont-want-epoxy-to-stick/