Apply clear sealant over old semi-transparent Behr stain without stripping?

I would like to add that the look is not very important to me.  All I want to do is preserve the deck as long as possible.  I don't mind a natural or greying look.  Just want to maintain the deck as long as I can, without putting poison in the environment and creating too much work for myself.

I live in San Francisco a block from the ocean.  The deck gets sun all year round, but we have a lot of fog!

No, you cannot apply a sealer over a deck stain. Try pressure washing and then using a solid color stain over the Behr:  https://www.deckstainhelp.com/what-are-the-best-solid-color-deck-stains/

Thank you so much for your quick response.  I am afraid to use a pressure washer as it could possibly damage my deck?  What about using the Gemini Restrore-A-Deck kit, which is what you recommend with a strong deck brush? 

Also, since the current stain is a semi-transparent stain, can I use another semi-transparent stain? 

What about future maintenance?  Will it be sufficient to clean once a year, and re-apply a semi-transparent stain every three years thereafter?  Or, if a solid deck stain is what I should be using for sure, can I just restain it very three years or so? 

I am trying to avoid having to strip the stain in future years.

The only way to use another semi-transparent stain is to fully sand and or strip off the Behr. Pressure washing is the best prep for a solid stain. You can use a cleaner while doing this.

Do I use only a cleaner or should I also use a brightener.  Based on your reviews, I am planning on applying a RAD solid stain.  I contacted RAD, and they said their stripper does not take the Behr stain off, and they cannot guarantee that the solid stain will work over the Behr.  Usually they recommend stripping, but in the case of Behr that does  not work. 

You can cover the Behr with the RAD solid stain but not their semi-transparent stain. Use a deck cleaner and pressure washing. Make sure all the stain that you leave in the wood is not peeling and you will be good to go.

Thank you so much.  Is there a particular roller type that is best for applying a solid stain?

Any roller should work.