Q: If one was to sand the Poly finish and then shoot Nitro over it, wouldn't the finish cracking in the Poly soon propagate itself to the Nitro?
Good question, and... my guess is it must work - for a while - because its something that is done. Maybe its because the poly is then thinner, or the nitro gives and flexes more... or maybe the the neck has now settled a bit. The practice of overshooting has been used for a long time though.
Q: I Nitro a softer finish than the Poly? If yes, then maybe this is how it would not crack when applied over the Poly ... but I thought (and maybe incorrectly) that Nitro was a harder, less flexible finish than Poly
Nitro tends to get harder with age. In fact it gets VERY hard with age.
Makes me think, if Warmoth was having problems with nitro necks coming back there was some issue that they were unable to master. Either one of moisture content and stability (I tend to think that that is not the case), or one of application/prep., or one of materials selection. The line of reasoning is that Fender and Gibson both make nitro finished necks, no issues. Lowly refin folks like moi (and countless others) do necks in nitro, and its takes a looong time for problems to develop. So I'm wondering if something they did, or used, or used for prep (ie, sealant) was at fault, or if the nitro they used was of the right consistency. There are many different blends of nitrocellulose lacquer.