Depth, This product is rated for interior use - not exterior exposure. While many people will use wood as a form for concrete, it seems that wood that is rated for exterior use is preferred; however, depending on how many times you plan to use this fiberboard circle as a form, you might be able to protect the wood and extends how long it is useful, with a paint that is approved for use on MDF. We suggest that you check with a PRO in the paint department at Home Depot for those particulars.