Hi, Rube. If you are going to just paint again, there isn't much that you can do. Any filler either would have nothing meaningful to stick to or would be unpaintable.
The way to fix the problem is to strip the paint off the columns, including trying to get as much as possible of the old paint out of the crack. If you have a small rotary tool (Dremel or similar) and can open up the cracks for the next step, so much the better. You will want to get rid of any rot that becomes evident after stripping the same way, by removal of the rotted wood with a rotary tool.
Then get some automotive Bondo and fill the cracks. Overfill the area slightly with the Bondo and sand it back to flush after it has set up. Remember that you only have a few minutes of working time with the Bondo so work quickly or mix a new batch.
By stripping the columns, you will bring back the defination of the turnings and the columns will look much better. By fixing the cracks and removing any rot, they will last a lot longer. Repaint with a good primer and then paint.