Install new sill if you can it is usually attached with screws.
If you remove the paint with time the aluminum will oxidize.
To prevent this and keep it shiny, get paint remover masking tape a cheap brush and a small bag of stainless wool get the one with different sizes and get a spay bottle of clear coat in the paint dept.Also you need rubber gloves. Small wire brush.
Protect all area with masking tape , shake well remover before opening and also open covering with a rag , be careful , I always open the bottle first to remove gases you know that build inside the can, close back and shake, then brush a little at the time to no overflow you will see bubbling after few minutes paint could be removed , brush again the remover and continue cleaning the done wash with soap and water finish with de-greaser rinse well , dry very well cover surrounding area use masking tape old news paper, shake clear coat well 3 minutes at least follow this to get better results, when painting about 8 ,10 inches apart from surface from one end to the other pass once one way and once the other , let rest 5 minutes repeat al least 5 times to get good thickness , let dry. Avoid windy days.