Going to be redecorating the house soon as I've just retired. The gap between window sill and frame, is it normal to fill this with caulk after painting?
If it timber on timber, caulk should be fine. Smooth it with a wet finger.
Assuming that you will be painting, do not use silicone. The paint will not adhere.
If it is a painted timber sill that meets uPVC, consider a MS polymer such as CT1. It is almost as a flexible as silicone but can be painted provided that you first apply a coat of waterbased paint.
That said, if it is uPVC, you can still use caulk but you need to make sure that you paint the entirety of the caulk- otherwise the unpainted edge/lip will become dirty.
BTW, my preferred caulk is Everbuild 125, a great caulk and very competitively priced (from £1 to £2). Second choice is Dow Painters' Mate. Double the price. CT1 is up to £14.
CT1, you can dip your finger in white spirit to smooth it. It is rather sticky so if the windows are uPVC, you might want to use tape on the uPVC first.
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