Could you add a building with the opposite of Sink? Like DeSink?
The only difference would be that it would automatically lower the quality of the item by 1 tier, always with a 100% chance.

Example: If a building random produces a Rare quality item, but I need an uncommon, I would just lower the quality to the desired one.

I don't have factories and a chain for higher qualities, so I won't use higher qualities, and if I had a mod for more qualities, I would have a chest full of one type of item with only X qualities. And I don't want that.

So I think that "DeSink" would definitely be useful not only for me :)

oh that will be easy since i have the required tech for that, would you always just want one of an item, or does it feel fair that converting an item to a lower quality gives 2 or more items back?

It would be completely enough if it worked like a Sink. Just that each cycle would lower 1 tier down. The last one would be "Normal" and it wouldn't destroy it. Normal items would just cycle until I removed them through the filter.

It would probably be enough to just have an object into which I would insert an item with quality and it would automatically decrease by 1. But since you have a finished process for increasing qualities, you could "duplicate" it and just modify it slightly :)

As for the convert ratio, you can easily add this to the settings for how many items it should return from better quality to worse. I would leave the default at 1

would you prefer it to be an entity that outputed just that, a fork of this mod entirely, just a new input belt with the downgraded items still appearing in the shop, anything else?

I'll leave that up to you :)
I would only have these 3 requirements.

1) I want to insert an item with quality somewhere and somewhere it would return an item 1 quality lower. And I could repeat this process until I get normal quality. If I do it with normal quality, nothing happens and I get back normal quality.

2) Possibility to set in the configuration how many items will be returned after quality reduction. Although I will use 1, I can imagine that someone might want more.

3) So that it works for qualities from other mods as well, not just the 4 basic ones.

It can easily be 1 item into which an object with quality is inserted and it will automatically convert it to 1 quality lower. It will not do anything for normal qualities. It depends on what is easier for you.

sweet thanks for voicing your preferences, i'll see what i can do (the process itself is easy, but i want to make it at least somewhat good for performance), i have some other modding projects lined up too so don't expect anything before tuesday

Cool. You can easily make a red Sink that will do the opposite or just a special new shop where if I put an item with quality in it, I get a lower quality. I'll leave it up to you to come up with what you want.
Here's another mod that exactly describes the problem I'm solving, but it only works for vanilla quality or has some other flaws - Quality Down-Binning
Then just ping me and I'll test it ;)

hmm could you join my modding discord? a bit easier to exchange prototypes there

It says the discord invitation is not valid, so I can't. But I'm directly on the Factorio server

are you sure https://discord.gg/ktZNgJcaVA does not work?

also ive just completed porting Quality Down-Binning to support additional qualities, all within a 25 minute timespan :D

probably yes, the link returns me this - https://support.discord.com/hc/en-us/articles/360001556852-Invalid-Invite-Links
I haven't tried that mod, as soon as I read that it doesn't work for other qualities, I moved on :D

really weird, i am pretty sure the link is valid

anyways i've pinged you on the factorio discord, let me know if that mod works for you, there's a link to the port there