Easy Sail Repair With 3M 5200

I had a couple small holes in an old mainsail that needed patching.  Over the years I've used sail tape as well as had holes stitched up "properly" with uv thread and Dacron material.  I stumbled onto a few websites where people had patched their holes with 5200 and scraps of old sail.  So for less than $10 I thought I would give this a try.  I went to a local sailmaker and picked up a small piece of Dacron that was in the trash.  The 3M5200 was ordered online for about $9.  The sail is over 30 years old but in moderately good condition.  I wouldn't do this kind of a repair on a new sail or a sail that was in great shape.  But to get a few more years out an old sail, why not?