How can I repaint a bad paint job?
The basic process I would use is  


Find out what type of paint was used (both primer and top-coat) so that I can obtain and apply a compatible paint. If I couldn't find out, I'd try overpainting a small area in an inconspicuous position and see what happens.
Sand it flat using increasingly fine grades of sandpaper, then clean it.
Buy new paint and carefully follow the instructions on the tin. Make sure to use primer first if there are any areas of bare wood.


Worst case is probably buy paint stripper and clean it back to bare wood.