Supplies Varathane Polyurethane for sealing joints?

Polyurethane is indeed a type of resin, another common one being epoxies. They differ in that polyurethanes are more elastic and they "dry" rather than use a hardener to cure.

You can coat resin with a Polyurethane sealant and it will be perfectly fine. Polyurethane is even more UV resistant than epoxies so it will protect from yellowing. Luckily polyurethane, unlike epoxies, can be removed with rubbing alcohol; so they're honestly not a bad idea for a sealant. I would of course suggest testing coating and removal on spare resin to ensure you get the finish that you want and don't have any issues removing it, as each type of poly may be a bit different.

Also consider that polyurethane come in both oil and water-based. Water-based is easier to use and clean-up, and it dries faster. Here's some info on the difference: Water-based vs oil-based Poly

Now, this is all general information I found searching online, but your question piqued my interest so I wanted to share what I found. I have only ever used oil-based poly for coating wood. I happen to still have some and now I'm considering testing it on spare resin; cause I would also like to try a sealant that would be a bit more sturdy at the joints.

 

Be careful to read all of the information on the label and check the reviews. Most polyurethane turns yellow.

 

I would not recommend oil based poly, entirely because I know oil based paints can get into the resin, and or impossible to remove, and I would be concerned about polyurethane getting into the resin and being impossible to remove in the same way.

 

I will preface this with that I don't have any experience with the specific product you mention, so take this for what it's worth. I did a quick search, but I don't know witch one you are thinking about (there seems to be several products within that line) and I don't know if it's a resin varnish or not, so read with that in mind.

And, yes do read the label, but never ever rely on the label only. There is not enough space there to tell you all you need to know. Always read the Safety Data Sheet to learn about what the product really is and what it contains, so that you know how to protect yourself, what to expect from the product and how to manage spills and leftovers.

And, to clear up the terminology, to help you know what you are reading when looking for info:
Resin is the liquid form of (among other things) a number of plastics, polyurethane among them.
(Think of it like this: cupcakes are made from cupcake batter, but the finished cupcake is no longer batter, it's a fully baked cake. Likewise, a BJD is cast from polyurethane resin, but the cured doll part is no longer resin, it's a piece of fully cured polyurethane.) I know people in the hobby often confuse this, but sloppy terminology when working with the materials is a serious health hazard.
It bears repeating, it is factually correct to say that a BJD is made from resin, but it is not correct to refer to it's current material state as a finished doll to be resin.

About resin varnish:
So, you may add a polyurethane resin varnish to a piece of cured polyurethane and it may or may not bond. If it does bond, they are now one piece and you have permanently added a layer to the surface of the doll.
To get a good bond on a doll that is not freshly cast, you may need to sand the area to make sure there is a fresh surface to adhere to. If a piece is heavily oxidized (yellowed) that may act as a barrier and the varnish may lift or flake over time.
Personally I'd hesitate to use a resin varnish of any type on a doll, unless I had extensive experience with the specific product. I'd like to have a general idea of what surfaces it works on, how it ages, if it shrinks, how fast it yellows, how it reacts to chemicals likely to get into contact with the doll etc. The SDS will help with some of this, but I don't think I would risk permanent damage to a doll without personal experience to back it up, since variables such as working temperature and humidity can make a huge difference, especially with polyurethane.

About other (not resin) types of polyurethane varnishes:
There does exist polyurethane varnishes that are not resin varnishes. Instead of being used in a liquid form, the polyurethane may be used in it's cured from, in tiny particles used as a (mostly) colorless filler. Same concept as acrylic dimensional varnishes and similar, just a different material.
These will not bond to the surface of the doll, but will behave more like a normal sealant. If it's removable or not, if it reacts with the doll or not and how tough it is will depend mostly on the varnish itself, the liquid that the fillers are suspended in. There are probably about as many different formulas out there as there are makers, but the general hobby wisdom still applies. Avoid oil based products. If solvents are used, make sure to keep drying times short. Use appropriate mask, gloves, safety glasses etc. as needed. Read up, make tests... you know, all that.

If you do decide to try it out, please let us know how it goes and how it holds up over time.

 

I've used brushed-on acrylic/polyurethane artist-grade varnishes to further protect surface blushings/painting like nails, "tattoos", etc. But how far into the joints are you considering? There's a difference between preserving paints/blushing on parts that will only be stressed by tight clothing and on parts that could be hit or rubbed by other parts (e.g., inside of the legs) or be subjected to stronger rubbing and scraping (e.g., the very edges of knee, thigh, elbow, shoulder, and smaller joints). Repeated abrasions will tend over time to remove whatever coating is used, and the paint/pastel/pencil underneath.

I was into polymer clay jewelry at one time, and we regularly used pure liquid polyurethane coatings (actually, the best and most cost-effective was plain old Future floor "wax"!). But jewelry and beads were not intended to be subjected to the kinds of contact-mishaps bjds are subject to. Nor are many polymer clay figures intended to that either. Most artists just try to ingenuously work around the limitations in their surface designs. Perhaps only advanced dyeing techniques might present resistance to surface wearing-down.

 

Using something very soft like cotton flannel to suede the joints could be a much better way to avoid fighting entropy and physics trying to stop a friction joint wearing away.
That said, I have begun dyeing my dolls with a light gradient on the joints to give them some blush color that will be a lot slower to wear off. It's tricky but it can be done if you're the methodical type.