After a few years, there will be little cracks and maybe some warping in your wood. This is completely normal, because wood is a natural, living material. Solid wood has the great benefit that you can sand away the top layer to make the surface smooth and flat again. You can do this every two to four years with a hand-held sander. Do it on a dry day. Use 120-grit sandpaper for this.

• Clean the table thoroughly with Extremis wood cleaner (see video) and let your furniture dry out completely (for at least 24 hours).
• Protect the coated or galvanized metal parts by taping them off with strong tape.
• Start by gently sanding any vertical edges until they are smooth, using a sanding block or hand-held sander. 
• Then sand the entire tabletop evenly to obtain uniform results.
  Move in the direction of the grain of the wood and hold the machine completely parallel to the surface. 
• To finish, sand the edges of each wooden segment by hand, holding the sanding block at an angle of 45°, likewise with 120-grit sandpaper. 

Once all the dust has been removed, this is a great time to oil your table using Extremis wood protector. Check out the instructions here