I just discussed this issue with Tunch and here are his thoughts.
Refreshing Nitro Seal can be done 2 ways. With POXY or with Part B which uses alot of the same base components in POXY.
He also mentioned several people getting extended durability with Nitro Seal with applying 2-3 very thin coats of Part A prior to Part B. I have yet to try this, but will certainly be giving this a try.