I am probably the board's DIY BKT expert--self proclaimed, of course. I have been DIY BKTing for nearly 4 years, using only QOD. I used to post quite frequently, helping out when I could until someone pi$$ed me off by saying no one was helping them. But I'm back...for now...I'm trying to find someone to split a bottle of Kerapremium with me because I'll never use it before it expires. I tried the Organiq and didn't like it. Fortunately, I was able to sell it.
I suffer no breakage from QOD products. In fact, my hair is past BSL. If not for my blonde highlights, it would probably be hip length when straight, but the highlights are killer to fine hair.
I am a super baby fine 3c/4a naturally. I began with original Max, then switched to original Gold, and have been using the Gold ever since. My curls are currently 2b-2c. I wear my hair curly. I, too, have problems with curl retention now, but I have a regimen that gives me full, billowy curls, sometimes even 3rd and 4th day curls.
First of all...I hope you're not continuing to do full-head applications. I only do root applications now. I haven't had to do a full-head application in a couple of years. I continue to use the 450˚ setting on my curling iron during the application process and have suffered no appreciable thermal damage.
If root volume is the problem, try not to take the keratin and heat all the way down to the roots. Leave a bit of curl there for volume.
The other curl killer for fine hair is oil--any kind...yes, argan oil, too. Clarify your hair and try products with NO oil...not a little oil...NO oil. Whenever I use oil, my hair is horrible...droopy...soggy...ugly. When I don't, my hair is fully, curly, bouncy, shiny, amazing. Every other day or so, I cowash and rinse-out condition with V05 Extra Body Conditioner. I have highlights, so I use a dollop of Pureology Reconstructor, but mostly for the fragrance. Occasionally, I use the clarifying shampoo that came with my first bottle of QOD to remove product buildup. Don't worry about "washing out" your keratin. It doesn't work like that. I even used shampoo with salt crystals in it--no problem, except that it had lots of oils in it, so it went to the product graveyard.
Next, I detangle with a Curlisto finger comb, scrunch in a small amount of a mixture of Suave Captivating Curls Whipped and Pantene thickening mousses, then plop in a microfiber towel for up to 2 hrs. I remove the towel and apply Curlisto Structura lotion and John Frieda hairspray and then pixiecurl (blowdry hair upside down in a bowl diffuser) until my hair is dry, adding more hairspray along the way wherever needed to lock in the curls. My hair is 80s big for about 30 mins following this regimen, then the curls begin to relax and cascade down my back. I can refresh by scrunching with slightly wet hands, respraying with hair spray, and drying/scrunching with the diffuser again for 2nd and sometimes 3rd and 4th day hair.
I use all cheap stuff on my hair (except the Pureology, but I make that last forever) and my hair looks more amazing than when I used to spend hundreds on Kerastase and a bunch of other obscenely high-priced brands.
I think the main success factors here are (other than the BKT):
John Frieda hairspray
Hope this helps...anyone!