Day 1. Prepare the rind:
Cut the watermelon into 3/4-inch slices. Trim the rind away from the pink flesh; make sure no pink is left on the rind. Trim away the outer skin. Cut the rind into approximately 3/4-inch cubes. Toss with the pickling salt in a large bowl and refrigerate overnight.
Day 2. Prepare the jars and lids:
Wash all jars and lids thoroughly with soap and water and rinse well. Fill your canner with enough water to cover the jars by at least 1 inch and bring to a simmer. Using a pair of canning tongs, lower the jars in gently, tilting them to fill with the hot water. In a small saucepan, keep some water warm but not boiling; place the lids in the water. Have an additional kettle of water on to boil.
Pickle the rind:
Rinse the rind thoroughly and drain, twice. In a large stockpot, combine the vinegar, sugar and cloves, and bring to a boil, stirring until the sugar is completely dissolved. Add the rind to the brine, bring back to the boil, reduce to a low boil and cook uncovered until the rind is translucent, about 20 minutes.
Fill and close the jars:
Using canning tongs, remove the jars from the canner, carefully pouring the water back into the canner. Set next to the stockpot with the pickled rind. Turn the heat under the canner to high. Use a ladle to pour the pickled rind into the jars through a canning funnel, leaving 1/2-inch headspace at the top. Run a clean chopstick around the inside of the jar to dislodge any trapped air. Wipe the rims of the jars with a damp paper towel. Place the lids on, and screw on the rings until just finger-tight.
Seal the jars:
Using canning tongs, gently transfer the jars to the canner, taking care to keep them vertical. When all the jars are in the canner, there should be at least 1 inch water covering them; if you need more, add water from the kettle until the jars are sufficiently covered. Bring the water to a full rolling boil, and process for 5 minutes.
Remove and cool:
Using canning tongs, gently remove the jars from the canner and transfer them to a kitchen towel or cooling rack, again keeping them vertical. Do not set hot jars directly on to cool counter surfaces. Leave to cool, undisturbed, for at least 12 hours. If any of the jars do not seal when cool, reprocess using the method above, or refrigerate and use immediately.
Label and store:
Add a label to the lid or side of your jar, noting the date it was canned. Remove the rings and store jars in a cool, dark place for 3 weeks before opening. Jars can be stored for up to a year. Refrigerate after opening.