weed killing in rain

Is there ANY weed killer that would work on say, cracks in paving or along paths in all this wet weather? I mean how often can you spray if you have to have 4 hours of dry weather afterwards!!

Have tried boiling water and boiling salted water and so on which is fine but if I don't kill some of the larger stuff soon they'll push my paving up!

I also have now got Ground elder, which i manage to keep on top of in the dryer weather most years, coming across the flower beds and marching off into the grass and beyond!

Which means I can't put my grass cuttings in the compost. :(
Janet Tweedy