How to Paint Unpaintable Caulk. There are two kinds of caulking in this world: caulking you can paint and caulk you can't. Basically, latex-based caulks are paintable while silicon-based caulks are considered unpaintable. However, everybody makes mistakes and one of the easiest to make is using the wrong kind of caulking. If you ever make the mistake of using silicon caulking in an application you want to paint (around a window for example) or you've moved into a home where a previous owner used upaintable caulking, before you start digging out that unpaintable silicon caulk, you might try one of these ideas. You may just save yourself a lot of work.
Rub a cloth dampened with mineral spirits or paint thinner over the silicon caulking. Allow it to dry and then apply two thin coats of an oil-based (alkyd) primer. Be sure to allow the first coat of primer to dry before applying the second.
Once the primer has dried, apply your chosen paint color, again allowing the first coat to dry before applying the second.
Alternatively, use rubbing alcohol on the silicon caulk and after it has dried apply two coats of shellac. Once the shellac has dried you can apply your chosen paint color.
Use latex caulking right on top of the silicon. Apply a bead of the latex caulking, then use your fingers dipped in liquid dish soap to spread it thinly over the surface of the silicon caulking.
Once the latex caulking has dried, you are now ready to paint it the color of your choice.