None of those concerns are valid,this is done all the time. I just wouldn't hire it done by someone that thinks it will look bad because if the last one they did looked bad yours will to.

First we need to see the issue and you need to get the right contractor who acyually knows how to do this. Post pics of the house here in a comment so we can see what you have going on. DO NOT start another post. If painting is what you are planning then there are other options for wrapping posts .

Why redwood? Cedar? Pine? Unclear why that specific wood. Please post a photo of your home.

Don't use redwood unless there is already redwood on the home.

"BeverlyFLADezinerDon't use redwood unless there is already redwood on the home."

I am a life long student always eager to learn so I must ask why using Redwood trim is bad unless there's already some on the house? Undoubtedly somebody with a kool French Boutique name holding a sign proclaiming to be a pro knows what's best. IMO Redwood is ideal for weather resistance, has as little shrink-swell cycle as any, better rot resistance unfinished than most and excellent paint holding ability. Until solid 1x become scarce and prohibitively expensive then finger joint started using cull I often specced it for facia and various other trim. It never crossed my mind there could be negative consequence from using Redwood unless it had been used elsewhere.

I am aslo a sign holder and agree you do not just plunck something anywhere without connection to something and I think this OP is getting bad advice from a "designer " who admits they do not know much about exterior design . Maybe you think differenetly than us so be it. But I stick to my comment about wanting to see the house firstly and if the OP is planning to apint there are options much better suited to that than redwood,

Here is the sum total of what OP said/asked.

Is it a bad idea to wrap porch posts in redwood?

"HU-53334342yesterday

He have two pretty skinny wooden porch posts. Our designer (who doesn't have much exterior experience) is suggesting to wrap them in redwood so that they are thicker. I have gotten conflicting info from carpenters. Will the seams look weird if we want to stain the wood rather than paint it? (The seams won't be right on each corner.) More importantly, will this crack and get damaged quickly due to water exposure and as the wood expands and contracts? We live in the SF Bay Area. Thank you!"

Nowhere do I see them ask if there are other ways or superior ways of making posts THICKER. Nowhere do I see them ask if posts should or should not be thicker.

In relation to Redwood they ask (A) Is it a bad idea? (B) Will seams look weird stained Presumably based on what carpenters say,OP asserts that seams won't be right on each corner. (C) Will Redwood crack due to water exposure,expansion and contraction? One only needs to go aboard 50 year old yachts to see Redwood which has endured more than any home ever will and still beautiful in their owner's eyes.

Why in the world do ya'll need pictures to answer those questions? Don't interpit me as coming down hard on designers for no reason,to the contrary I'm attempting to show how you can do your job without causing needless conflict with and amongst other tradespeople during construction. Many many people are clueless to how they want their home to look and call on designers to basically decide for them and are happy in the end knowing their look is the latest. Nothing bad about that and I wouldn't suggest homeowner do it different. Where problems arise is when a designer assumes others are the same and proceed to call shots without asking. Still no foul or harm,,,,,,,until,,,,,,a real live example that follows. Builder furnished in house pre-contract consultants,engineer, architect ,decorator consultant,material package and trades subs for new house under construction. Homeowner hires a decorator consultant that has no affiliation with builder. After home is in the dry but long before cabinets are delivered decorator convinces homeowner the kitchen island would look better and be more practical at a different position. Hard feelings ensued over who would foot the bill for relocating plumbing and electrical buried in slab. I blame the homeowner for not just backing away and leaving plan as it was but they didn't and flames flared when concrete contractor insisted an addendum be attached relieving them of warranty due to demo plumber had to do to foundation in order to relocate plumbing. Eventhough costs for change orders fell to homeowner,decorator faned the flames by telling homeowner electrican could bury junction box and house wiring with plumbing to avoid a hideous floor recptical where it wasn't needed. With everyone on the job simultaneously soured, the inspector retaliated by giving plumber a bad time over not being able to avoid connections below and/or incased in slab. I tell this story and there's some that are worse,to point some things out. Do decorators stand behind their work with implied or issued written warranties same as other trades? I've never known anyone in construction other than decorators that routinely cause change orders for issues they aren't qualified to perform or do a detailed material list and scope for.

Context is everything in design. Yes we need pics of the house . Very often when OPs come on here they are asking one question but need some help to understand why we answer the way we do. BTW most yachts were teak not redwood . Often OPs do not know what they do not know that is why they come here and IMO as pros it is our job to make sure they do know what they are asking .and get the right answer.

Will the seams look weird if we want to stain the wood rather than paint it?

(The seams won't be right on each corner.)

What do you mean by 'seams'? As you asking about fingerjoints?

If 'seams' = 'joints', how can you not have joints at the corner?

As to redwood--excellent choice for exterior if you can find it and if you can afford it. If all you can find is finger-jointed material, it won't look good with any translucent or semi-translucent stain. Plan to use either solid color stain or paint.

Totally agree with klem1. Many responders in this forum have a bad habit of asking for more information than is actually needed to answer a simple question. I would strongly suggest to the OP that they NOT post any pictures of the house. Pics would be totally irrelevant to the question asked and will lead down a rabbit hole.

Redwood is resistant to rot and decay, which is the reason it is commonly used on exteriors. Redwood is not, however, usually painted. If you plan on painting the posts to match other parts of the house, I would suggest another wood. Also, of course, paint has a limited life span and will need to be reapplied periodically. Finally, whatever you do, do NOT miter the corners, since miters inevitably pull apart and take in water.

Why redwood? Because it's the default choice for exterior wood in the SF Bay area. That's why.

And we totally paint it out here. Or we don't.

Well then I guess just use it and we can all relax. There is always more to questions that need some clarification and why would you ever paint red wood that makes no sense .

Thanks, everyone! Attaching a photo of the house in case it's helpful. What I meant by seams is where the pieces of wood wrapping the posts are glued together. I understand that solid redwood itself is a resilient wood, but my question is about when redwood boards get glued together around an existing solid post. My concern was that, as the wood expands and contracts, cracks could appear in those seams relatively quickly. Or water could get in and cause damage. One experienced carpenter warned me of this. Secondarily, I was wondering if it would look weird if I didn't paint them but just stained them, because the boards would not all align on the corners, so there would be "seams" visible. I don't know the right terminology and am new to home projects, but just trying to figure out whether wrapping would be a bad idea. Thank you!

Your carpenter could always do a lap joint. That is a common way to join wood. It gives you more surface area for the glue to adhere, and it allows expansion without exposing what is underneath.

My neighbor recently wraped his metal posts in cedar and it looked very nice.

With the photo now provided, I'm not seeing how a simple wrap is going to work & look good. It appears that any wrap would leave end grain exposed where the post meets the beam. The overall thickness of a wrapped post will exceed the beam thickness. How to work the devil out of the details is a big question. Yes, the posts can be bulked up with a wrap but how is it going to look overall? Without bulking up the beam, too? And how to bulk up the beam where there doesn't appear to be enough space to do so on the fascia at the drip edge? Just bulking up the posts is going to look like a job half completed, IMO.

ETA: Gluing redwood is tricky because the wood is naturally oily. You are in using polyurethane or epoxy glue territory. Application and cleanup has to be fastidious if you intend to stain the project. Not a place for Titebond glue and a swipe with a wet rag.