There are a few folks that have HDG frames. I do not know their specific results. I saw one high-end builder claim they did it.
@mudgudgeon and @Malleus are both correct.
From my 40+ years of fabrication experience (Industrial sheet metal and structural mechanical equipment), whenever we Hot Dip Galvanize (HDG) structure, we must allow for warp and twist. It can be significant. Yes, the 815°F to 850°F is correct. Depending on the alloy of steel, the annealing temperature of the metal can range from 1350°F to 1770°F. The closer you get to those temperatures means that the "strength" and "ability to flex and return" will be affected. This is metallurgy and how you work steel and use the alloys and heat treating to alter the flexibility and strength depending on application.
Also, the "shinyness" of the HDG can vary greatly based on how old the zinc is in the vendors tank. A fresh tank can produce some parts that are very shiny and look new. A well used tank will produce parts that are a dull gray and look like they have been weathered for 20 years when they just came out. It is totally a hit or miss based on the vendor and how often they change their tank.
If this were my own frame, I would NOT HDG it due to the severe risk of warp and twist. Now, if money was no object and I had direct access to a frame machine and all data, I would consider it, knowing that it would take a considerable amount of care to get the frame back to straight and square.
Powder coating:
If this was a show car only that will never see rain or dirt, I would consider doing powder coat. In order to PROPERLY do powder coating, ALL edges, corners, and points MUST be rounded smoothly in order to keep the surface tension of the powder coating even during the liquifying process. Otherwise, the surface tension pulls the plastic too thin on the corners and edges and it will become very thin and it will fail on the edge. Once powder coat fails, it allows moisture under it (even just relative humidity) and it will continue to peel and flake, with no acceptable repair.
If I were to do my frame, I would blast it near white (SSPC-SP10) and then coat it with an inorganic zinc coating, then a coating of epoxy, then a topcoat of polyurethane or epoxy. This also ends up as a VERY thick coating (about 12 mils TDFT or 0.012" thick)
When we do equipment that is near the coast or in a "marine" environment, this is the system we use. This is NOT for immersion service, such as under water.
At the very least, I would do near white blast SSPC-SP10, then do two coats of Sherwin Williams Macropoxy 646, then a topcoat of a polyurethane or epoxy in semi-gloss black. John Deere uses Blitz Black and is a HD agricultural paint that seems to hold up well and is a gloss finish black.
Any epoxy you get should be a bonding epoxy. The key is in the material preparation and the prompt application of primer in a timely manner and in a humidity controlled environment.
