There is a marvelous solvent that is called THF. It is the major component in PVC pipe cement. This solvent will melt PVC and Urethane. You can use this melted compound to actually coat bare spots on a boat, or to help seal up pinholes in the material.
For Urethane, you must melt the coating off of fabric that you use for repair. This works well, because the color can be matched exactly.
You can also melt the coating off of PVC fabric. However newer PVC fabrics have a base solution coat that make them more air retentive. This base coat is usually white, and the white will mix with the other top layer resulting in a lighter coating if you try to get all of the coating off of the fabric.
For PVC we often recommend that a customer buy some vinyl tubing and cut it into little pieces in a Tupperware container, and then fill it with solvent until the solvent just covers the cut up tubing. The resulting mixture looks like transparent spaghetti O’s. Devolving of the materials must be done 1 day prior to use.
If you intend to strip the coating from the fabric, acordian fold the material strips. Place the folds on edge, and allow them to sit in the solvent for 2 hours. Use plastic gloves (food handlers gloves made of polyethylene) to remove the fabric form the solution. Simply grab the fabric and pull it between your fingers (with plastic Gloves on) to squeegee the softened plastic off of the fabric back into the Tupperware container.
For small repairs here is an alternate method. Hold the fabric in a concave shape, and pour about 10 drops of THF in this material. Use a nail to scrape the melted coating away so you can get an exact color match.
Remember that this solvent will evaporate fast. Keep it in a covered airtight Tupperware container. Always be careful with solvents. Do not use them around open flames like water heater pilot lights. Wear Goggles. This is strictly an outside activity. Keep these solvents away from children.
Thin solutions of this material (Tomato soup consistency) with multiple coats. Will give the best results. This consistency will have enough solvent in the solution to melt into the existing coating, yet will not dissolve all the way through the fabric to the other side before it dries.