The old finish should be thouroughly cleaned, then lightly sanded all over the whole surface to be recoated, or treated with liquid deglosser. It's best if you use the same type of finish material each time you recoat, but not absolutely necessary. If you're not sure what the existing finish is, then test with the new finish material by applying it in an inconspicuous place to be sure it won't bubble or have any other adverse effect on the old finish. Clear Finish Material Spar varnish is a very good exterior finish, as well as my old sandby, lacquer, either brush on or spray . The spray cans of lacquer are easy to use and make the job a lot quicker and easier.
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If you find it necessary to strip your patio furniture it would be a good idea to sand as necessary since anywhere that the old finish may be missing and exposing the bare wood, the wood fibers will probably be kind of rough. A good finish for patio furniture, starting with bare wood is, Seafin ship n shore wood sealer, a deep penetrating sealer which is resistant to moisture and then coated with spar varnish , or with seafin teak oil. Both spar varnish and the teak oil are moisture resistant making them good for outdoor furniture. Lacquer is a good finish too. Use a lacquer sanding sealer on the bare wood, then coat with a brushing lacquer or spray lacquer.