You can waterproof your indoor cushions to use outdoors. Doing so is simple, but requires regular maintenance to ensure the long life of the fabric. Cushions covered in sturdy fabrics, such as heavy cotton or durable polyesters, will fare best outdoors.
Spray On Treatment
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Waterproofing spray is found online and at stores where you find home and garden products. These sprays are formulated to bond with specific fabric types to coat your cushions with an invisible and impermeable sheath of protection. Some sprays are heat activated, creating a stronger bond with the fabric for longer lasting protection. Most treatments require reapplication every season.
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Wash In Treatment
If your cushion covers are removable, you have the option of using a wash-in treatment. Add the waterproofing solution to your wash cycle to coat the cushion covers inside and out. This approach could affect breath-ability of the fabric. Also, you'll want to wipe out your washing machine afterward to remove any residue.
Considerations
Clean your cushion fabric before you apply any treatment. Dirt will undermine the effectiveness of any waterproofing solution you use. Consider using a treatment to prevent UV damage. Companies like Trek 7 offer spray on products that are easy to use and will keep your cushions looking new for longer.