Let me elaborate.
Current paints do attach themselves to the plastic door panels relatively well. However, it will not last forever and if you hit it right the paint will come off. Whether this happens right away or in a few years...it will happen. The plastics that are on new car bumpers are formulated to accept paint adhesion. 1970's door panels were not formulated for this. They are polyethelene, which does not allow for great paint adhesion.
However, if you have a color interior, it's probably your only/best option.
My point was, if you have black panels, just buy another set. Black is the easiest to get. I personally wouldn't go through the hassle to paint black panels and then worry about matching gloss with the rest of the interior and also worry about paint adhesion. However, maybe the budget doesn't allow for this.
Also, if painted well, you won't fill in the grain. However, I have seen some panels where they were painted too heavily and it did fill in the grain.