Is there a paint-on primer that perhaps would not clog/fill in tiny PE screens or air vents? How about painting (brush painting) methods that won't clog those same tiny details/parts?

I've had good luck with Mr. Metal Primer, which is a clear liquid. I brush it one and then immediately go back with a clean brush and punch through any holes or slots that have been filled in.

I would think that if your paint (for your finished color) is thin enough, using the same method should work.

Yes Mr Metal Primer is very good indeed. I sometimes dip small PE parts into the bottle directly with a tweezer rather than brushing it on.

Also, sanding the PE parts with a fiberglass stick will help the paint or primer adhere better, by removing the oxide layer from the surface.

Automobile self-etching primer works wonders.

For painting engine grilles, You could try misting the parts from a distance, at a low psi, slowly building up paint. This won't fill up the holes in the engine grille. For me, Vallejo surface primer is great, as long as you spray the grille as mentionned above.

I concur

thanks. I don't have an airbrush, hence the questions.

I switched to Army Painter primers a few years ago and now I use them on everything. It sprays on easy and smooth and dries in minutes and works well on PE as well as resin. Screens such as porous parts aren't an issue. They come in assorted colors and the bottles are rather large.
Terry

I use Tamiya Fine Primer in the rattle can, and I have just started using Alclad Primer, which requires an airbrush, but goes on super thin, and provides greater control.