on dull window frames i find if that on the first clean you just wipe ,using CIF cleaner on a damp wet rag ,then on the next 2 cleans just buff with scrim, they come up shiny. the passage of time between each clean is important. residue from the CIF takes weeks to work its way into the pvc . take a while to explain this to your customer ,its a long process ,oh yes, but nothing good comes in a hurry !
its similar to using an olive oil bath to soak old coins in,takes weeks to do its job