It's probably peeling and cracking due to many undercoats of paint already on the sill. For best results, remove all the old paint (or as least all so that whatever is left is just a thin film of color). Then prime and paint the sill.
if the paint peels, that's due to water infiltration of the jamb. it suggests there is leakage or weeping under the siding to me. and to let the water out, that means latex primer and paint.
you should probably moisture test several areas of the siding to see if you are setting up for something worse than a scabby looking window sill. how are your gutters? are penetrations, seams, and window frames well caulked? ice dams or water creeping along under a sketchy layer of shingles?
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Posts: 5849 | Location: North Burbs, MN | Registered: Mar 14, 2007
Zinnser makes a product called Peel stop. We use this in these cases. You must have no paint on the sill. sand completely. Try to apply it in cool weather 2 coats. We then use Kelly-Moore brand Dura-poxy paint. It's for interior/exterior and has UV protectant in it. We always apply 2 coats at least.
Ron's wrapped suggestion is to have them covered with aluminum rolled stock. A "tin man" would use a brake to bend aluminum roll stock to the profile of the sill and cover them.
Any advice given here is general in nature and is not necessarily valid for your given area. If in doubt check with your local codes enforcement department for what is required when doing electrical, plumbing or structural work on your house. Permits may or may not be required in your area and home owners may not be able to DIY some tasks. I have no way of knowing if you have the skills needed to complete the tasks you are asking about, when in doubt seek professional assistance.
My advice may be worth exactly what you pay me for it. :-) For the record I did not stay in a Holiday Inn Express last night.