Spirit-based, clear preservative works for me - the only slight trick can be remembering which bits you've done and which you haven't, because it's clear.
I've used the above treatment - quite a lot of it - recommended by @Woody2Shoes and found it good. Agree that it's a bit of an issue knowing where you've treated because it's clear but, when aplied, you can just about see the "wet" bits.
I’d supply it bare with the provision that the customer is to finish. Anything else opens you up to a potential claim that your pretreatment wasn’t compatible with the subsequent finish. The nightmare world we live in.
I've had good success with water based preservatives. shake very well as they are an emulsion. allow to dry properly before painting. £22 for 5l sika from screwfix. these days I avoid solvents amap.