Contractors experienced in using silicone caulk and paint will tell you that you must prepare your caulk for painting or your paint will not adhere to the caulk. They will also tell you to choose carefully your method of preparing your caulk for paint.
Safely Preparing Caulk for Paint
To prepare your caulk for paint, avoid using a power sander. It won't work. It is best, in fact, to avoid sanding your caulk at all. You should rough it up to receive your paint or primer, but instead of sanding, rub the caulk surface with a cloth and denatured alcohol.