What next after trying oxalic acid on scum line?

One side it did decent job, the other side not so much.

I tried 3-4 passes, scrubbed it in, let it sit, and nothing to get some patches out.

I still have some bad spots as shown in the picture, so wondering what my next step should be. Muriatic acid next? i know that stuff is POWERFUL, so trying not to go that harsh, but maybe i have too. Only other consideration was using a "gel-based" oxalic acid cleaner. Starbrite has some and FSR is gel based. 

been holding off on muriatic acid because i'll need to wax after...  but i finally think i know what to do, and am going to order orbital buffer, pads, compounds, polish/wax to do this on a weekend.

I'm no expert but it seems to be going the buffer, compounds route before cleaning is kind of backwards and going to end up kind of a mess.  I'd give a good cleaner one more shot.  This question comes up a lot on boating forums.  There are always 100s of great answers, a few of which might actually work.  You got the household cleaner (like CLR) crowd.  Forget it, not close to strong enough.  You have the tidy bowl crowd, I dont know, maybe.  The hardware store acid crowd, sounds like the route you went, again, maybe.  Then there is the specially made boat cleaner crowd...like me.  I like Mary Kate On and Off Gel.   Not a chemist, so I dont know the difference, but hydrocloric + oxalic + phosphoric) Unlike all the light liquids that splatter and splash and dont stick, the gel sticks to the side of the boar (and the bottom, ugh, but I do it every fall).  Let is sit for about 10 minutes, hose it/wipe it off.  Stays in the water from April until Oct/Nov every year and by the time I put it away, it still looks new.