I had my bike clear powdercoated. I had to do most of the preparation myself. Preparation consisted of having the frame chemically stripped, then sanding out some of the spots missed by the strippers. (!). I sanded it with wet and dry paper, used dry and started with 180 grit and finished with 600 grit. It is a very tedious and messy business. I sanded for about 30 mins. a day until it annoyed me enough to stop. Every time I stopped sanding for the day, I sprayed the frame with WD40 to prevent rust. It will rust very quickly if this is not done and while the rust may not be immediately apparent, it will show later after the powdercoated is applied. I then stored the frame in a large plastic garbage bag until the next session.
When I took it to the powdercoaters, I cleaned it with denatured alcohol (in the powdercoater's foyer) and wiped it with a clean rag then after carefully masking it with powdercoating tape I placed it in a fresh bag and and handed it over. I made sure that the powdercoaters understood to treat the frame with care and not leave any fingerprints on it, although I've since considered leaving a fingerprint under the bottom bracket for ID and as a sort of signature.
I'm reasonably pleased with the results, although,with the benefit of hindsight, I could have done better.
If (and when) I do this again, I would spend more time on sanding. I sanded this one until it looked polished, but the coating dulls the sheen a bit and the surface is duller than it was before coating. I would probably buff it using a mechanical buffing wheel to bring it up to a chrome-like finish, which I know is a lot of work (done it on my hand-tools). I would also specify a double coat of powder. Like all powdercoat, the edges of the lugs will not be as defined as they would with wet paint, but being clear,it won't show as much as an opaque colour. Powdercoat is pretty tough, but it's not invincible and it can still be chipped or scratched. Those chips and scratches will rust instantly, but clear nail polish does a good job of covering them. I would also invest in a set of small riffler files,to clean up the brazing a bit.Incidentally, if the brazing is polished a bit, the powdercoat preserves the sheen. It looks a bit like fine lug lining.

I live in a fairly dry climate and don't ride the bike in the rain...ever. It may be different where you live. The finish has been on the bike for a year now, with no rust apparent, except where I have put nail polish on the very few tiny scratches. I used a tiny piece of abrasive paper to get those few small specks of rust out before putting the nail polish on, and the rust hasn't returned.

It really is essential for a top class finish to get every tiny scratch or file mark out before coating (something else I will do next time). The clear finish will show marks which are barely visible before coating.

The obligatory pictures without no thread is complete...







Good luck with your project.