You can use a de-glosser - it's the consistence of water. Just wet a rag and wipe them down. This goes by lots of different names - de-glosser, surface conditioner, liquid sandpaper, etc. All paint stores (not DIY) can help you. It's cheap and very easy to use.

Thanks Kitty....that sounds easy. I will check it out. In the meantime, I have seen some black chairs at Ikea for $19.99...hmmmm !!

Mineral Spirits will take the gloss off the lacquer.
I don't know if sanding is really necessary. I would definitly prime them so the paint will coat evenly.

Oxalic will help you to clean up the wood and scuff the finish at the same time. You find this in wood (exterior) cleaning solvents and it is also inexpensive as Barkeeper's Friend sold at super markets (where you find cleansers in cans like comet) Use gloves and clean up the chairs well, then prime and paint. Sand only where the finish is loose or too shiny so the new paint might not stick to it.
Flood makes a product called Wood Cleaner sold at HD.
There is a lot in the gallon container it comes in, but you can clean you deck and all exterior wood with this as well (before painting)