I am not very familiar with Rimguard so hopefully someone familiar with that particular product will chime in as well.
I am very familiar with leaky tires.
I keep a spray bottle of soap solution handy to help find leaks. Commercial bubble solution is probably the best, but mine usually ends up being a bit of laundry or dish soap and water.
If you have a really tough to find leak, there are commercial bubble solutions at a heat and air supply store such as Johnstones.
Anyway, spray down the tire, including the rim, rim weld, valve stem etc. Then step back and let it sit a moment.
Rotate (drive) and do it again.
Sometimes it takes patience, sometimes it shows right up.
I would think with the fill you should see a damp spot on the tire where the leak is if you parked on concrete or such.
Anyway, keep at it and your leak should show. You can also de-mount, steal your kid's plastic pool and dunk it holding it under and then sit and wait for a while looking for the bubble trail.
We usually go through about 100 dog turds (brown plugs) a year. I like the heavy brown master brand. Plug and go, multiple plugs if needed. This year has been great so far, only about 10 flat tires
I am guessing you don't have many flats, and with the fill I would locate the leak, make sure what / where it was and pull the wheel and tire with it marked in crayon or sharpie and take it too a commercial tire shop and have it patched inside.
Most filled tires (especially the old CC fill) they would put in a tube before filling but not sure what the standard is with the rimguard. If there is a tube in there (close examination of the valve stem should tell you but sometimes that is tough) be careful how you evaluate a leak around the valve stem.
Good luck hope this helps.
