get product custom field after checkout in php

For my product I have 3 custom fields:

Brand
Model
Size

These are product custom fields.
How do I retrieve these values in the controllers/checkout.php file? They are not in the full order when I do this:
Only item custom fields appear. Any way to get this working?

Hi,

Product custom fields are in the "product" object. You have to load the product using the product class.
In the order, you will have the "order product", it is a different kind of product and it contains "item custom fields".

The data come from two different tables. Because the product custom fields are not stored in the table "order product" you have to load it directly in the table "product".

Regards,