Repairing the wood.

Since this is a safety item, I would replace the rotting wood. To maintain the new wood I would use an oil stain on it at least every other year to keep the wood from rotting.

You can always patch it, sand and then paint. This will hold out for a couple of years but eventually you will have to replace.

Beyond repair. Best to replace.

Please tell your family not to touch the railings. Terrible nasty splinters

The filler products are expensive and probably should be used in places where replacing boards would be difficult (window sills, garage doors, etc.) A deck railing would be an easy and fairly inexpensive replacement. We recently dug out rot on a wooden retaining wall and filled with cement that we were using elsewhere. It was easy to paint over and will buy that wall many more years.

Smooth it down with a wood plane and apply a clear protectant such as Flood CWF

I had the exact problem with lots of splinters. I used a belt sander to smooth the top surface then I painted on several layers of exterior wood glue (TiteBond II) until the surface was smooth. Mostly, the glue was used to fill in the low spots and in the process I applied one layer to spots that still looked good to keep the wood look. Then, I painted it with good exterior paint. It has lasted thru two wet cold winters and looks the same today as the day I did it!

Thank you, Linda. I will follow your suggestion. I appreciate your help.🙂