Hello and thank you for using Just Answer.Regulation 1524 (2) PAINTING, POLISHING, FINISHING. Tax applies to charges for painting, polishing, and otherwise finishing tangible personal property in connection with the production of a finished product for consumers, whether the article to be finished is supplied by the customer or by the finisher. Tax does not apply to charges for painting or finishing real property.
In short yes, the powder coating would be subject to CA sales tax.
Here is a link to the complete sales tax regulations for CA.: