Cracked lawn rollers leak, making them less effective at flattening and compacting an area, due to the reduction in weight. Fixing a crack with adhesives, caulks or body putties works, but it is not a long-term solution. These patches fall out or crack rather quickly. Cracks in steel are the result of metal fatigue, which needs to be relieved by welding and normalizing the cracked area.
Drain the roller and allow it to dry for several days.
Place the roller at a comfortable work height, in a cradle, on ramps or on a workbench. Position the roller so that the handle is on the side of the roller that is opposite the crack. Place blocks against the roller to keep it from moving.
Clean the area around the crack to bare metal using a wire wheel on a right angle grinder.
Use the highest heat recommended for welding the metal used to make the roller barrel. This information may be found in your owner's manual or in a product description on the manufacturer's web page. Fill the crack with weld, beginning about 1/2 inch before the crack and ending 1/2 inch past the crack.
Immediately heat the welded area with a propane or oxy-acetylene torch until a 2-inch area all around the weld has turned from black to dull red. Allow heated portion to cool to room temperature over the course of an hour or more.
One cool, use a 24-grit wheel on the right angle grinder to clean and smooth the welded area.