It takes many generations. Breeding Lavender to Black (without Lav lineage) is the route to fix it, but it's not a quick fix.
Ideally, one would start with a large batch of Lavenders to choose from, and pick only the cleanest feathering.
Then cross to Black. Hatch a lot, because only a few will be worth breeding.
Match up those siblings, or put them back over a parent.
Rinse, Lather, Repeat.
You have to keep a line of Blacks free of Lav, because the black offspring that are split to Lav can also carry the shredding gene. Don't cross to other, shredded Lavenders.
Shredding comes in degrees, so you want to pick the least shredded. You won't get totally clean feathers in two or three generations.
It's honestly a lot cheaper to pay for birds that already have clean feathering. Some breeds don't have that option. I could not find any English Orpington breeders offering Lavenders with clean feathering.
On the other hand, Ameraucana breeders have put in the time and work, so nice clean ones are available.
