I painted 3 new sills a couple of years ago but the paint, particularly on the one that gets most sun, is blistering and flaking off.
If I ever get around to painting them again how do I get the remains of the old paint off to leave a smoothish finish?
When I bought an old terraced house I used a blowtorch on the doorways to strip them back to the bare wood. After this I gaily vacuumed up the old paint, and then leter couldn't understand why the vac wouldn't work.........
I find a hot air stripper is much easier and safer to use than a blowlamp.
However if you are working anywhere near upvc you need to be extra careful not to ever ever let the heat touch said plastic as the results are irreversible.
But as you are working with a concrete substrate then I'd recommend wire-brush-and-soapy-water appraoch and absolutely only use paint and primer product specifically for use on stone / concrete.
A liquid / colloidal paintstripper like nitromors will also do the job but same conditions apply re contact with upvc. Also very nasty to inhale the fumes released.
Masonry paint such as "Weathershield", Sandy. You will have to get back to bare concrete though. Not an easy job, but I'm sure you're no stranger to elbow grease ;o)
I did Tony....................and the vacuum repairman thought it was hilarious when he had to ungunge all the clogged together paint from the vacuum hose (I actually blamed it on my mother)