Houses that have window sills coated in varnish may require stripping if they have been exposed to moisture and the sun over a period of time. If your windows are exhibiting varnish that is peeling and/or stained then you will need to use a paint stripper to remove the old varnish before re-coating with fresh varnish.
Using a small brush, apply the paint stripper to the window sill liberally as shown in Pic 1 and allow to stand for about 10mins or until you begin to see the varnish blister and peel. With a scrapping tool as illustrated in Pic 2, begin scrapping the dried paint stripper/ varnish off the window sill. Depending on how many layers of varnish were initially applied and how deteriorated the varnish has become, you may need to apply the paint stripper several times and scrape accordingly until all the varnish is removed. When you have completed removing the varnish, give the window a light sand and wash the window sill with mixture of one part Oaxilic acid to four parts water if the window sill exhibits any water stains or weathering. Remove any excess Oaxilic acid with fresh water and allow for the window to dry. Once fully dried, the window is ready for fresh coats of varnish to be applied as displayed in Pic 3.