Can I Seal Over an Old Stain?

Washing with just water is not a good idea. The use of too much pressure will damage the wood. It is always best to use wood deck cleaner or a stain stripper. In your scenario a deck stain stripper will "soften" the old coating so you can wash it off at a low pressure.

Make sure to neutralize the stain stripper with a wood brightener when done.

It seems like the HD80 deck stain stripper will work great for my deck. It's an old semi-transparent deck stain that I am having trouble removing with just water and pressure.

I wasn't aware of having to use a wood brightener but I will now that I know. It makes sense to brighten the wood back to the proper pH before sealing it. This helps a bunch, much appreciated.

You are welcome!

Deck Strippers and Deck Cleaners are caustic cleaners that will raise the pH balance of the wood. This "darkens" the overall appearance. Wood Deck Brighteners are mild acids that neutralize the caustic in the wood, lowering the pH to a neutral balance. This step "brightens" the wood.

Never wash wood with just a power washer as excess pressure will cause damage to the deck.

This is somewhat technical but the way you've explained it makes total sense. The right cleaners allow you to use lower pressure and brightening is necessary after using the right cleaners, easy enough.

I will attest to excess pressure causing damage. There are some areas of my deck now that are furry for a lack of better word. I got a little aggressive trying to get the old deck stain off. I guess better late than never to learn the proper way.

You can lightly sand the furry wood with a sander. It does happen with stripping or heavy cleaning. If it is not too bad try a scotch bright pad ( the green side of a spoung under your sink)