Depending on size of the flaw, I have had good luck patching with Polyspan. I apply the patch with Ambroid and apply a light coat or two of Ambroid to fill. Follow that with a light touch with the sandpaper and I doubt you will be able to see the repair. Silkspan works almost as well but takes more to fill and is not as strong. Polyspan is great stuff!

Polyspan is critical about which side of the material is "out", be sure you apply it properly.