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How to Repair a Cracked Composite Door Even though composite doors are extremely robust, they may still develop cracks due to general wear and tear. These cracks are generally easily repaired. All you need to do is use a plastic filler and a UPC-friendly cleaning agent (HG uPVC's powerful cleaner is ideal). You'll also require medium or fine grit Sandpaper. It is essential to first clean the crack. Identifying the Crack Composite doors are a popular choice