I tried the deep clean procedure yesterday and after 30mins it was still all kinds of nasty.
Drained the filter and found a bunch of twig bits and pine needles.
As I shop vacuumed the heavy stuff off the top(wouldn't float with the hose on) it was evident how deep the debris went.
Realized what probably caused this mess.
Stopping and cleaning the suction basket after vacuuming and then switching to backwash pumps a very small amount of the stuff that doesn't get caught in the basket during removal into the bottom of the filter sand.
Multiply this by who knows 30yrs? and the sand is pretty much filthy.
$60 for 300lb of sand sounds like a better option that trying to sift it, plus I'll get to check the fingers.
Hindsight I kind of wish I'd have done an FC test off the filter drain after letting it sit overnight to see how much the organic material was sucking up. In all likely hood it was saturated with chlorine but who knows?