Have a deck made with old PT 2x6’s that needs paint or a thick stain. Not looking to replace boards at this point, but I’ll have to do some sanding to get old loose finish off. Any suggestions for a product that can be rolled or brushed on that won’t start peeling off in less than a couple of years?

1. Wear proper PPE sanding PT lumber...dust is toxic and an irritant.
2. Nails and screws, if used, in top of wood will tear up any attempts to sand - suggest a good cleaner and pressure wash <15,000 psi to not tear up wood.
3. You will be hard pressed to find ANY stain/treatment to last more than 2 years on old PT lumber - find a good semi-transparent stain.

I have used WoodRX for years and I am very pleased.

https://woodrx.com/

If the under structure is solid, prob better to replace the deck instead of gouging out the surface layer with high pressure and then sealing that with some polymer. You'll be back in the same boat very soon afterward. Might be less headache to just replace the deck surface boards (depending on any complexities related to existing railing mounting method etc). Weigh the relative cost, effort or headache, simplicity etc. of replacing vs extending the life of an old deck that may already be near or at end of life.