I had good results previously painting the sidewalls with a few coats of Sadolin Superdec. A tester pot is only a few quid but I can't remember which colour I used. Superdec wears by erosion rather than peeling/blistering but don't try heavy jet washing. It lasted a good few years for light use with an extra coat now and again. It's waterbased and dries in an hour or two in good conditions. A few coats will help bond the loose threads together but better to run at higher/moderate pressure as constant sidewall flexing at too low pressures will soon deteriorate again. Take care when fitting to prevent excessive folding/flexing. Clean the tyre as much as possible before application, I managed to paint around the logo which was still in good condition. I think i did 4 coats which made the sidewalls much smoother visually. I wore the tyre out in the end but it's still at M&D's somewhere. Will post a picture if I find it but I've got a hardly worn pair of Ground Controls with stringy sides which I was going to give the same treatment. Ok with older tyres which are drier but I imagine adhesion on newer tyres will be reduced due to the oils in the rubber compound.