I still do it the old way, i grind the paint away, then spread my plastic filler over bare steel, then I use NCP 271 Corrosion Resistant primer as it has the etching chemicals built into it.
Been using it for 4 years now and no problems yet.

Don the way you are doing it is just fine, if your not having any problems and comfortable still doing it , by all means keep doing it.

Spray