Well if you don't want to buy an electrical gadget then it will be very hard work. Either use paintstripper to take it all off and start again or use sandpaper on a block - both very time consuming. The easiest is to buy a small electric sander, they are not expensive and always come in handy.
All this of course if you want a smooth finish at the end.
Jack I bought that some years ago and it never worked - the guys in the DIY told me something like this that the very strong stuff that was in the paint stripper was removed thru health and safety.
I'm in the mode Islay because of the lovely weather I am going to do this - the sills are quite low. So perhaps of my reduced mobility will go for the wee leccy sander
Mahogany unit turned out absolutely beautiful. I had bought wrong paint ages ago - but neighbour had the perfect paint.
Then I have sanded and scraped all the window sills. The electric sander was no good and so I had to use boiling kettles of water which loosened the paint. I had also bought the paint remover (bottled stuff) absolutely hopeless and I had hears some years ago they had removed the real macoy out of it that really lifted the paint. I had bought two bottles so brought one back.
At the mo taking a little rest as they have been undercoated and glossed (front). Will be going to back of house to finish off the gloss. I feel satisfied LOL