have you taken off the rear bumper and rear arches near the bottom? there most likely to be rotten to if the sills are, mine were anyway.
I think it doesant help with the plastic trims as they hold in the water, i bought i mig welder in the end for £140, A sealey mighty mig 100 gasless, done the job nicley, i then applied multipurpose grease with a paint brush to all the arches before refitting the trims to prevent further rust.
if you like mk2's its worth doing as most are rusting unless there being looked looked after, i wouldnt bother with a mk3 there worse for rust, at least its not visible on a mk2
