Yes...for even penetration, color, & wear of your NEW stain, sand through all existing finishes.

* Sweep-off the bulk of sanding-dust.
* Shop-vac (with the bristle nozzle!) to remove more dust.
* Wipe with paint-thinner dampened rags to lift out the last of old material.

Now...you've basically got a new deck again!
* Most importantly, the wood WILL ABSORB & HOLD STAIN EVENLY because all sun & weather-degraded wood is gone.
* What remains now is sound, open cell-structures...NOT crumpled/crushed fibers!

Besides...sanding is chemical free!!!

I went through ~ 14 gallons of Sikkens Semi-transp. SRD Redwood, #089 last yr...@&%^WY$& fence took most of it!!

I sanded all my spindles, steps, and skirtboards last yr...Uffda! They look NEW again now though!
(It's 12 yr-old Redwood in Fargo, ND)

Is your deck low or high?
Decks that are on the ground, or less than 3' high don't get good air ventilation underneath. Solid stains don't breathe as well, and therefore have a higher tendency to get "pushed-off" from ground-moisture vapor moving up.

Prep & wood-dryness are big factors too though!

Faron

Thanks Faron. I'll sand then. My deck is about 10 feet above ground for the most part:

I'm still on the fence as far as solid color or semi. My understand from this forum is that one of the cons with solid color is that it peels and cracks vs the semi which tend to just fade in color. But consumer reports shows this Flood solid color as the only stain that resists color change, and peeling. We are going to move out of state in 2-3 years and I'd like this to be the last time I have to stain this deck. Why not go with the solid color? The good thing about the Sikkens srd is that only one coat is needed but it's $35 a gallon. I can get the Flood for $20 a gallon but I'd need more of it since I'd probably put two coats down. The flood is a latex, film forming stain which according to this forum, is bad and should be avoided?

I have switched to solid color stain several years ago. I used the Behr deck and fence stain. It has never peeled, just faded. I reapply a new coat every two or three years. I just pressure wash the surface and apply with a roller and it looks great.