Are you confusing scumble with the likes of Floetrol?
Scumble will extend a wet edge but is designed to act as a carrier for tints. Floetrol is designed to provide an extended wet edge BUT without diluting the paint.
The stuff I used was called scumble in the deco merchant as far as I remember, but it wasn't the glaze stuff, used for "wood graining" which was some sort of spirit- soluble liquid, I think. It was like a wetting agent and only needed a small percentage so no really diluting. Floetrol looks ideal.
The effect was superb with Kitchens and Bathrooms paint- sort of emulsion with a hard skin. Made an orgasmically perfect even eggshell finish. You know, you run your fingers over it and go "ooooooh". No?
Anyway ask at a proper deco merchant, NOT B&Q , who sell something like "traditional acrylic ......" surely an oxymoron.
I had a go at fancy paint effects at one time, with flogging brush, flitch, sponges and whatnot. Can look good, but a bit of a fad. I showed someone some ragging on a raised/fielded panel cupboard door a while ago. Dilited paint, rag, about 30 seconds - woman thought I was Michaelangelo.
You reminded me to look at my French polishes - Button, Garnet, Van Dyke, Piano and the rest. All solid, now in the dustbin which is probably illegal. I used to enjoy that.