I did some testing with knife blades (shiny smooth surface) as phosphate has harder time adhering there.
I used my MnCO3 based solution from earlier.https://imgur.com/lNss3g3
"1" was only cleaned with acetone.
"2" cleaned with acetone, phosphated, phosphate removed with H3PO4 diluted solution, rinsed with water and phosphated again.
"3" Cleaned with acetone and half was kept in ~10% H3PO4 for approx 10 min (wanted to keep for minute but forgot). From acid straight to phosphate path.
500ml MnCO3 solution from 10l tank + 500ml i had from earlier combined + added 6g MnCO3 to reduce some acidity.
When solution was hot PH was 2.2 ish. https://imgur.com/lu3uawX
left - cleaned with acetone.
middle - acetone + few minutes in dilute h3po4
right - acetone + half in dilute h3po4 but here i rised hot solution PH to 3 with NAOH.
Surface activation/preparation has big role in end result.