Can used headers be ceramic coated inside and out and if so, what company would you recommend?

Yep and Central Connecticut Coatings. Gene has been doing my headers and powdering my parts for 3 decades.

Who is Central CT Coatings?
Gene Schaeffer
President
gschaeffer@centralctcoatings.com

Born and raised in Boyertown, Pennsylvania. Gene graduated from Millersville University in 1976 with a degree in biology. His career began as a chemist developing high temperature coatings. In 1987, he was hired by High Performance Coatings™ (HPC™) in Manchester, CT as General Manager. In 1994 he co-founded AirBorn Coatings Inc.™ In January of 2006 AirBorn Coatings™ CT changed its name to Central Connecticut Coatings Inc.
Gene has 30 years experience in coating formulation and coating processing.

A lot of guys here use LO-KO Performance Coatings
Never heard of a complaint. Been in business a long time.

If you have FB.

https://www.facebook.com/lokocoatings/

HPC in Burlington NC has been doing coatings for a long time and a lot use them. They do great work,lots of choices on the finish. They are not cheap but then quality rarely is.

I don't know who Dynatech uses, but mine are coated inside and out as well.

Performance Coating of OKC did a well seasoned set of headers and intake for me. Very durable.

https://www.header-coatings.com/
This is who coated my headers. They can do used or recoat headers that have been coated. They have a tumbler to clean them with.

Thank you all for the info. HPC is closest to me so that is where I will go.