I've used the diluted acid in a cheapo spray bottle; i found it easier than an acid brush. The acid will eventually eat away the spring and the bottle won't work anymore, but it worked long enough for me. Just spritz it on, wait til it stops bubbling, then brush with an old toothbrush to knock off any loose pieces of scale. Rinse and repeat!
I recently had a couple of pools with a really bad tile line (I mean reaaallllyyyy baaadddd). I tried dilluted MA and all I would up with was some lightening of it in some areas with obvious "drip" lines on some areas of the tile. I thought I was hosed. I then got some Biodex 300 and a BBQ grill scrubber, got in the pool(s) and applied some elbow grease - and the tile looks (almost) new.