It’s not actually a deck. It is a pergola cover. But the question is the same (i.e. wood exposed to the elements).
Do I need to sand off the old sealant/stain, or can I just seal over it? It’s been several years, and I think I need to refresh the sealant/stain to help preserve the wood. It always has been “rough”. Not finished smooth like a deck would be. So there really isn’t an aesthetic reason to sand it.
If it was furniture (or similar), I’d strip the old and apply new. But for an outdoor pergola cover? Is that really necessary? I’m just trying to protect the wood.
This is tricky. I had a deck that was made from pressure-treated pine - not today’s wood. This was built in 1979 with tight-grained copper sulfated pressure-treated yellow pine. Sealed initially & probably about 1990ish. It was painted to sell in 2002 - when we bought it. The deck had southern exposure in Nashville, TN. Wood grain had opened up between growth rings - not affecting structure, but enough to allow water to enter and further compromise things. The house siding was T1011 which had rough sawn cedar as the exposed surface - not smooth. The siding was certainly painted when built 1979 and again to see in 2002. Again I expect once again roughly 1990ish.
The cedar siding was fine. More than a few deck boards had to be replaced during the period from August 2002 until 2012 - the year we moved from Nashville.
I don’t know your materials or maintenance since construction - those factors and the importance you attach to appearance are likely the most important factors.
An edit - forgot to include - the wood had been stained - not solid stain or paint by the previous owner. When I noticed water was damaging the wood as it entered the open grain, I changed to solid stain, which is really just a flat exterior deck paint. That would seal the deck boards until the winter snow and cold opened the grain each year. I never did try sanding or grinding off the upper deck surface.
You can sand pressure treated lumber, but it’s generally a bad idea. A really bad idea. PT lumber is filled with chemicals - an arsenic compound in the old days, today copper derivatives, but which are still harmful to health (that’s the whole idea: to keep out insects and other life.)
If you sand, you should wear respirator packs, go as shallow as possible, etc. etc. If the wood has been stained, then you can’t get to it without going into the actual grain, because unlike paint, stain penetrates.
If you must, strip, then re-cover with whatever you’re going to do. Solid stain, paint, etc. If you can, just take care of any flaking or loose material, clean, and put another layer on top.
I’m not certain it is pressure treated. I am aware of the chemicals, and have pressure treated lumber elsewhere in the yard. Actually, in the garden before I knew it was potentially hazardous. Doesn’t seem to have harmed anything. But you do raise an excellent point about sanding it.
So, I’ll just climb up and reseal it over the original sealer (or what’s left of it…I suspect not much, but haven’t climbed up to check how bad it is).
FYI, we haven’t really done any maintenance on it since sealing it. It has concrete pillars, and the coarse 2”x12” boards forming a “roof”. The boards are mounted edge-wise, and there is a spacing of about 12” between “slats”. It provides shade most the day except when the sun is such that it shines through the slats (roughly noon every day). It hasn’t shown signs of needing any maintenance, but I can’t see the top of the boards from the ground.
I did install electric on the underside a few months ago, and could see that the stain had faded significantly towards the top edges of the boards. So, I’m assuming it is mostly gone on the top 2” edges.
I verified with the builder that he just used Douglas fir. Not PT. But, as aesthetics don’t matter much (it’s a rough Douglas fir, not sanded), just sealing over everything may be OK. It’s not like I’m trying to sand to a nice finish. Just sealing the wood with a bit of a tint.
If you want it smoother, sand. If you just want the wood protected, apply your coating. That said, you will want to keep water out to avoid raising the grain of the wood. Something like Thompsons water sealer does a decent job of making the surface hydrophobic. Read the label for recommended coverage - under applying needs to be avoided.
For a short time. It’s basically a paraffin coating which wears off with rain. It lasts from 6-10 months, depending on weather. Then you have to reapply. A permanent coating is probably better.