I have a normal, double-size, bedroom quilt, filled with "duck feather and down" at 850g/m2. I have been thinking of emptying it and making an ultralight down camping quilt. Is there any reason it wouldn't be suitable?

My main reservation is that quill stems of the "feathers" keep poking through the quilt, causing some discomfort. So either I would have to sift out the down and just use that - or, I would need a fabric which resists them.

And (my 2nd question) - what would be the best fabric to use? It must be highly breathable, due to a dry skin condition. It must also be very light, and if possible, cheap ;)

You could use it, but it wouldn't be as light, compressible, or durable as a high quality down. I would guess that, at best, it's 500-550 fill power. A high quality down (800+ fill power) should have minimal quills and feathers. The quills in a lower fill power will puncture a lightweight nylon shell, and they may poke you uncomfortably as well as damage the shell.

Separating the feathers from the down would be incredibly tedious and time-consuming. Down likes to fly away, and it difficult enough to deal with when you're just dumping it from a baggie to a baffle. I really wouldn't recommend it.

At 850 g/m2, your quilt won't be very light. For my non-metric mind, that's 24 oz/yd2. That's more than my homemade quilt weighs altogether, and you'd still have at least 8 oz of shell fabric and baffles.

What you could do, since I'm assuming you're on a budget, is find a moderately lightweight down bag on ebay that you don't mind cannibalizing. I recently bought a GoLite Feather-Lite bag for $86 US that I'm going to turn into a quilt for my mom. There are instructions on this site, if you haven't seen them.

If you're determined to do this, just be aware that you will have a relatively heavy quilt, and you'll need a heavier shell fabric to protect you from quills. Down-proof might not be feather-proof!

It looks like my duck feather quilt is very low quality - those quill stems are far too numerous and too irritating. I've shelved the idea for the moment. Thanks for the advice.