I have a lot of door and window trim that I finished with a satin polyurethane. I recently started a project to finish trim in another location in my house. I somehow managed to use a different satin poly that, about half way through my project, I realized had a different finish. The can said satin as well, but it's clearly glossier than the other trim. So, can I use the old satin over the glossier finish. Should I sand it down, rough it up or something else?
It's unfortunately a common problem with different finishes that their definition of 'the same' surface finish don't match! Even gloss varnishes aren't all equivalent. This is because there's no standardisation in the finish industry and each company has their own standard for sheen on the ladder from matt to gloss.
So, can I use the old satin over the glossier finish.
I'm assuming here the two polys are the same type, either both oil-based or both waterbased, otherwise you would have noticed.
So should be no problem, different finishes of the same type are all fundamentally similar and this makes them fully compatible, allowing them to be physically mixed1 as well as used one over the other if desired.
Should I sand it down, rough it up or something else?
If the later poly is freshly applied it shouldn't be necessary, any more than it's necessary to sand or scuff between each coat of the same finish.
You should do this if there are any defects in the finish, e.g. dust nibs, but you're sanding just to deal with those and not to ensure adhesion.
Note: multiple layers of a satin finish cause a slight cloudiness or milky effect because the amount of matting agent increases with each coat, not a huge deal but sometimes noticeable. With just one extra coat it may be all right, but it's possible that three coats may not look quite the same as two coats even though the surface gloss level is the same.
It would be advisable to try a coat of the previous finish on a short section to see what it looks like, rather than commit to all the trim around a window or door and then find out it doesn't look as you'd hoped.
You can matt a finish down by abrading it, and you might be able to mimic the level of gloss of the first poly on the second by scuffing the latter down lightly. A fine grade of steel wool and Scotch-Brite (or similar non-woven nylon abrasive) are both good for this. Steel wool sheds little steel flakes as it wears down so it is messier to use and this could be an issue for work around the house2.
If the poly is still fresh enough it won't be close to fully cured and therefore won't be as hard as it will eventually be. As a result it can be scuffed with things that aren't commonly thought of as being abrasive, including the rough side of brown paper (common grocery bags are an acceptable source for this, thank you Bob Flexner). I've found that even coarse cloth can sometimes be enough. So again you can experiment on a small area and see what result you get.
1 Useful for example if you want to make your own reduced-sheen varnish from a can of matt and a can of gloss. Some waterbased finishes are an exception here as there may be pH differences from one product to another that means they can't be safely intermixed, while still safe to use sequentially.
2 Personally I find it easier to control the sheen using steel wool. But many prefer Scotch-Brite/equivalent and are perfectly happy with how it works for them, and it is most definitely less messy in use and lasts longer.