Is this happening on the inside or outside? It sounds like either the grids have paint on them that is oxidizing or bubbling or the vinyl is low grade and is deteriorating. You could try wiping them down really good and then applying more paint.

Thanks for the reply. It's happening on the outside. They were never painted, and I had a painter come out to look at them and he said you don't paint plastic. Is this true? If I try to clean them, what do you recommend that I use. It's more than just a light coating of chalky stuff.

You can paint plastic if it is properly prepped.

What color?

They are now white and I was planning to paint them white. How would I prep them for painting? There is a rather thick layer of the chalk.

Scuff pad, good washings, and wipe down with a tack cloth.

Kilz or Zin... the one with the bulls-eye next to the kilz cans - both primers are supposed to be ok for plastic.

"Kilz or Zin... the one with the bulls-eye next to the kilz cans - both primers are supposed to be ok for plastic."

The problem with painting plastic is getting a primer that is aggressive enough to bond well without being so aggressive that it weakens the plastic long term.

Plain water based products are rarely aggressive enough to bond well.

There is a product that I have used. It comes in a spray can. It is for certain plastics.
Check with you hdw store. It does not need primer. It melts the plastic and fuses the new color paint to the old.
Don't remember the name. It worked well. If memory serves me, the surface needed cleaning with TSP then acetone or MEK.
It might be worth checking into. FWIW Dave