Palm spots on fronds cure?

I have a couple of Chamaerops humilis palms. I understand they are prone to fungal blemishes. I have been spraying mine lightly every two weeks with Rose Clear, as advised (?). But the bl***dy new growth is spotty as well. They are in containers in a soil based compost, very lightly watered and fed. Any cure suggestions apart from a pot of palm green emulsion paint ?
TIA

 

Does it look like this.

This is on a small cerifera.
This grows next to a pretty large green bog standard one
It never gets the problem, though must admit the new growth on the cerifera is clean.

 

The spots are similar to the Google photo below. The website said it could be due to a potassium deficiency (?) Any ideas ?

 

I'm not sure @HarryS I would have thought potash was unlikely mainly because I always associate that with flowering.
I've got a couple of small ones in pots and I find they need more water and feed than you would expect to keep growing well.
My small one in the ground gets dry in the summer and pretty much stops growing.
The spotting seems to appear over the winter mostly.