looks like those blades won't stand up to being used around rocks? as for the bump knob wearing out, what kind of weed eater do you have? in some cases the bump knob can be replaced. chemical control is the easiest way to keep weeds down.

I agree with HU , also the bump head should not be allowed to be in contact with the rocks in the 1st Place . 6" of line is all that should contact the rocks and then only lightly , when properly trimming . I trim around 100 ' of curb and other ornamental cement structure and rock formation with no issues . Chemical control as advised may be prudent in your application .

I live on a three acre property with a variety of trimming challenges. Over the 35 years I have lived here I have gone through several various trimmer heads and found the best for all applications to be the Lee Valley Item number EC 717 as well as their line item number EC 720.

http://www.leevalley.com/en/garden/page.aspx?p=51146&cat=2,2160,40710,51146