BACKGROUND: Yes, I've read all the posts, here and elsewhere; about trying everything from silicone to duct tape. But after fighting tube flats over the years, and being envious of those with tubeless tires, I began to experiment with what might be an efffective spoke nipple sealant.
I sealed the rear spokes on my '00 Tiger 2 years, 20K and 3 tires ago. I sealed the rear spokes on my '05 KTM 10K and 2 tires ago. A friend, using the same process, sealed his Tiger spokes several tires ago. No problems at all, none expected. I just completed sealing the rear wheel on a '01 Triumph Bonneville (chrome steel wheel) yesterday.
ADVANTAGES: Easier to change tires; punctures can be quickly fixed with a string-plug. Also, normally water enters the spoke nipple area and over time causes rust/corrosion. This process completely seals the rim; no more rust/corrosion.
DO NOT try to seal 21" front wheels, or any wheels without safety beads;
those bumps that the tires pops over when air is applied. Those beads do two things: help seal the tire bead to the rim, and help keep the tire in place in case of a flat. Also, while rear tire punctures are common, front tire punctures are rare, plus usually the front tire lasts much longer than the rear. I expect bikes that are pounded off-road on a regular basis, are not good candidates for sealing, as they often need spoke replacement/adjustments.
MATERIALS NEEDED: wire brush or two, soap/water, a 1/2" tubeless tire valve stem, a drill bit to enlarge the valve stem hole (or use a bolt-in metal stem). I like the rubber stems, as they can be found most anywhere if ever needed. And one tube of sealant: Amazing Goop. Here's a link to the Goop site, so you can read the hipe. http://www.amazinggoop.com/amazinggoop/index.html
Goop comes in many varieties, all of which are the same stuff, in a different tube. Pick one. Most hardware stores carry it in one or more forms.
GETTING STARTED: Clean, clean, the spoke area to be sealed must be clean! Use a wire brush, pen knife, picks or whatever to remove any rust or other material from the spoke nipple area. Last thing is to wash the rim/spoke nipple area with soap/water/rag/brush, whatever it takes to get it completely clean. Then allow plenty of time to dry completely. There is no need to rough up the rim with sandpaper/powered wire brush, etc.
APPLICATION: This is a three-day job; DON'T RUSH IT! First day, clean and dry as above. Then set the rim on edge. Apply Goop straight from the tube to each spoke nipple; around the nipple, and over the center. Not too much, but enough to cover. Do only about 6-7 spokes at a time, then wait
15-20 minutes for the Goop to set up before turning the rim to do 6-7 more spokes. This is so the Goop won't sag as you turn the wheel. When complete, set the wheel aside for 24+ hours to cure.
SECOND DAY: The first application will have shrunk, and when it shrinks, it goes down into the areas around the nipple, giving a good seal. Now, apply a second layer of Goop, right over the first. Same as before; around each nipple end, and over the spoke end. Work under a good light and check your work, looking for any voids. Set the wheel aside for another 24+ hours to cure.
THIRD DAY: Recheck each spoke nipple. If all is well, drill out the valve stem hole to pull in the new rubber valve stem. (If you drill, remove any roughness or burrs) Or bolt in the new metal valve stem. Do not use a rim-strip or tape, or anything over the Goop. If you've allowed it to
completely cure, it will be quite tough now. Install your tire, using a good bead lube, like Murphy's Oil Soap, which is ideal. Inflate the tire to proper pressure, set it aside for a few hours, then recheck pressure before installing back on the bike.
If you've taken your time with this, and checked your work, there should be no leakage. But, if the tire has lost several pounds of air pressure, spray on a soapy water solution to find the leaky nipple. Then remove the tire to reseal the culprit. Clean and dry any area to be resealed.
Remember to add a string-plug kit to your tool kit. Also, string-plugs are the only plugs that work well with steel belted tires.