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The Free Form worked well for me to fill the gaps. It may be a bit time consuming to do it for an entire patio. I seems strange that the concrete is sticking to the form. I know i made my concrete quiet stiff (as little water as possible) without any sticking issues. its made it a bit tougher to smooth the top surface tho. You may want to try a few more water/concrete possibilities. Make sure the form is clean and maybe try something more aggressive than PAM like Vegetable oil.
Depending on where you live, its normally comes in either 5.5mm or 1/4 inch. I think what I used was 5.5mm. Any 1/4" plywood would work.
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