Depends why they are scabby surely? Injury, mites, allergic reaction, mallanders - clipping may make it easier to treat the symptoms but won't necessarily fix the problem. You need to know the cause and then decide.
My lad is fully feathered and until now I've never had any problems...BUT recently he started itching and a friend gave me some of this stuff to try!! It's amazing and worked literally within a day!!!
It's not cheap but worth the money to save the feathers!!
Not sure what the cob oil contains, but maybe something similar to pig oil and sulphur? I've used that successfully for mites and mallander. If he's not itching or stamping though, its probably not mites. Mallander would be high up (behind the knee) and not usually bright yellow IME. If it was me I'd probably want a vet to take a look.
Sorry didn't read properly so didn't see it was yellow (pus??? I'm guessing?) in which case I'd certainly want a vets opinion first. For general itching and the normal scabby dry bits heavily feathered horses get then definately try the cob oil, or something similar but I'd get an expert out first.