Fabrication and Welding of Plastics

Hey gurus, I have been asked to provide services for welding and repair of tanks in some industrial settings (galvanizing, pickling, etc.).

I am pretty good at the hot air or gas method and even taught it in an apprenticeship program many years ago.

Estimating for fabrication and fitting is a snap for me, but estimating weld times and cost are not my forte.

Does anyone know any links, books, hints, etc for the welding of PP with the hot air/gas method?

Here in the tri state area I find the companies doing it years ago have all closed or diversified and not doing it anymore as they got larger.

The ones who are still in business have taken the route of cleaner work.

Thanks in advance.