Sometimes, stuff just happens. You can wake up one morning, all primed and ready to go out and paint that fence in your yard, then come to a crashing halt when you realise that that funny noise you hear on your roof is actually rain. Since it is impossible to paint in the rain, you're going to have to wait until after the rain has stopped to fix up that fence. Painting a fence after it has rained isn't too different from painting a fence before the rain, but there are a couple extra precautions you'll need to take.
Wait for your fence to completely dry out from the rain. Depending on your climate, this may take several days. Paint will not adhere properly to wet or even damp wood.
Brush and scrape away all the dirt and grime from the areas you wish to paint. Rainwater contains dirt and particulates that will stick to the fence even after the wood has dried out, so you will need to remove these before you consider painting. A hard rain may also cause dirt to splash up onto the fence from the ground.
Mix up the paint according to the directions on the can.
Apply the paint to the wood in smooth, even up-and-down strokes in order to achieve a uniform texture and quality. Do not try to get it perfect on the first coat, as most fences will take at least two and possibly more coats of paint before they achieve that attractive quality you are looking for.
Give the paint several hours to dry -- exact drying time can be found on your paint can -- then apply additional coats as necessary.
Make sure you check the forecast before you begin preparations for painting. You're going to need at least two days of clear, dry weather in order to get the best result from your efforts. If there are any sections of fence you do not wish to paint, cover the edges with masking tape or painter's tape to prevent the paint from leaking over.