1- How do you know how much sealant to put in? I see anywhere between 30ml and 70ml being put in to 28mm tyres.
2- When you top up (every 3 months being mentioned) do you remove all the old sealant and start again or do you just add more?
1) I put 50 ml in my 28's, so slap bang in the middle.
2) Just changed mine this week after 8 months, the rear was ok-ish, the front was coral. I removed all the old sealant.
It was all much easier than I was anticipating, less than 30 mins to do both, quite impressed - I hate fettling my bikes. I did it without removing the valve cores, just unmounted the tyre, wiped out the old goo with kitchen roll, put most of the tyre back on, added the sealant using a purloined syringe with a bit of fish tank pipe on it fed into the last bit of open tyre at the top of the wheel, finished mounting the tyre and inflated first go with the track pump.
Lost a bit of pressure after the first night, topped them up and spun the wheel, they seem good now.