Go to youtube and search for your question as most large companies put out how to videos answering all sorts of questions, and teaching you how to do things.

I don't know if we have oil or water based stain on our deck, but we sanded it lightly when we reapplied stain so it would be absorbed.

you Can not apply oil over waterborne you can strip it and apply oil base stain after that

If you apply stains over each other without stripping them, they will create a shiny varnished look and probably peel. Always pressure wash and strip your deck first.

Or wind up paying someone like me a lot more money than you would have before to strip off two coats of stain. Storm makes an inexpensive chemical to spray on and pressure wash off your deck stain.Home Depot also carries an excellent deck stripper.

We ended up sanding down the waterbased (not durable) & applying FLOOD brand oil based. I think we got 3 good years out of it, compared to 6 months with water based.

Probably next summer we will re- stain.

Try Sikkens next time. It's been very good to me and my customers the past decade.