We need to determine how bad the sills are before we proceed.
Caulking on the sill or window that is pulling away a little in a few areas only requires a quick fix. Cut away the caulk that’s pulling away by using a utility knife. Be careful not to cut too deeply into the sill.
If when cutting out the old caulking you cut into the paint of the sill, sanding a little more in those areas will smooth out the lines and allow paint to fill in the cuts.
Next we fill in the gouges, cuts and scrapes that will not be filled with paint. A fast drying wood filler or nail-hole filler works great. Sometimes mixing a little warm water in with the filler makes it easier to work with.
Now a light sanding will need to be done to rough up the surface so that the new paint will adhere to the old paint. When light sanding, I use a 200 grit sand paper and putting sanding pressure just enough to rough up the old paint
We now need to caulk. I recommend a 35yr. siliconized acrylic latex caulk bought in a tube to fit in a standard sized caulking gun. Cut the tip of the tube so it’s only a hole about as big as a pencil lead. It’s a good idea to have a damp rag handy to wipe up extra and smooth it out after laying a bead down.
Now using the caulk gun, lay a bead down in the corner, sill to window and sill to side wall.
Continue to squeeze the trigger of the gun, smoothly, and slowly move to evenly lay the bead down. If it looks like you missed areas wait until you’ve gone all the way across the sill before adding more.
If I only missed a 1/4 inch or so, I wait until after I’ve wiped it with a finger or the damp rag. Wiping the caulk with a finger smoothes the caulk out and pushes it into the contours of the corner making a good seal. This makes a nice edge to paint to and seals out any drafts from the outside.
Now determine if you need to add more. In the skipped areas just add a little and wipe with your finger again. Sometimes I need to remove excess and that’s when the damp rag comes in handy, just wipe it off and let it dry for about 2 hours.
Now we can paint. You can mask off areas around the sill to not get paint on there, but I’ve found that in some cases the paint bleeds under the tape. Usually I didn’t find this till later when the paint has already dried. This will mean that I either need to scrape off the paint, paint over the paint or sand off the paint.
This is not effective or efficient, so I’ve taught myself how to do what is called “cut in”. I very carefully paint with a high quality appropriate sized angled brush so as to not paint anything I don’t want paint on. It takes practice and patience, but in the end it looks better and is much faster.
Now to painting, stir the paint up really well and make sure it is room temperature.
I like a 1 ½ or 2” wide angled brush. It allows me to see the work easier as I’m painting. I only dip the brush in the paint can, at the most ½ inch. Because I am right handed, I start on the left and work to the right.
Paint on left to right, smooth out the paint, right to left, lifting the brush before it hits the side sill wall. Keep doing this until you’ve reached the end of the sill.
Now either go to the next sill that’s ready for paint or wash your brush. Since I recommend water based paint, I always wash my brushes as soon as I’m done, using warm water. High quality brushes can be reused many times by doing this. Another option is a foam brush. I’ve not used them much but have heard that they work well.
If when looking at the finished product you see some areas that you missed, you can usually spot paint these areas. If you see a lot of brush marks, I’ve seen painter use a blow dryer while painting to smooth out the lines. You can also buy a product called Flowtrol and add the amount they recommend on the label. Sometimes heating up the paint can in hot water for an hour before painting thins the paint a little making it spread more evenly.