Many of my car modeling customers use the powders over gloss black or white all the time...they seem very pleased and thats what counts the most.
Rub n Buff was fine for its day, but I found it to be too labor intensive and difficult to clean up to correct any mistakes of flaws I found as I worked. Thats when I discovered SnJ...twelve or so years later...I bought the company.
If you have some powder left from your previous projects, try the gloss black or even over a silver, any of the Testors enamels will work fine. Best time to apply is just before it is totally dry. Remember when the Stooges would powder each others faces with talc using a big fluffy applicator...treat a cloth with powder and do the same...let it sit until the paint cures then you can start to polish it.
If you have any questions just drop me a note.