This might be a very bad idea, but I thought I would get some input from the knowledgeable people here. I’m near the end of a refinish project on a nice Ruger number one walnut stock. I have pretty much followed the recommendations of custom builders here, and used many coats of spar varnish, very thinly applied. I am wondering if a polyurethane topcoat would create problems, or be worthwhile. Thanks for any feedback.

I can't see it contributing anything to what you already put on it. Spar varnish is already a good finish. I would not do it.
Unless you just want to do it; first do a test piece.
The finish I use is a mixture of poly and tung oil.

Thanks dpcd. Of course the spar varnish has oil as an ingredient. Sounds like your mix approximates that.

Most spar varnishes these days are urethanes. So, I don't think you would be gaining anything.

My mix is sold by Brownells; I don't advocate the home brew stuff that some use. I rely on actual chemists for that.

Basic rule of thumb with those who have wooden boats is traditional spar type varnishes and modern polyurethane’s are not compatible with each other. They are very different chemistry.

Polyurethane’s are quick and cheap and look plastic. Traditional spar varnishes take time, really part of the wood and are long lasting. If you want high shine with a spar varnish let it properly cure and then polish it up with rottenstone and then carnuba wax.

On a gunstock knock back the spar varnish with 1200 then 2500 grit wet and dry. And then apply a few coats of a traditional oil type finish. You will then have a traditional finish without having to do 50 plus coats of oil.

I did a Idaho cowboy test on finishes, used all of Brownells stuff, and other finishes, used walnut wood 6x8" 1/2 inch thick, finished them and tacked them to my Idaho shop roof, for some hot summers and cold wet, snow, below zero weather for severall years as they lasted..who won, well that damn gloss glass finishes that most of us hate,

I settled for myself on Gun-Save-r from Brownells, for a number of reason, ease of application, weather proof, pretty, comes in satin or gloss and you can change your mind with either at any point according to application.

My tests showed most of the Brownells finishes were suitable, and that application itself was of more importance..

What handled 4 years on the roof? Old fashion Varathane in the black can..Comes water thin and takes two lifetimes to finish with a apply and wipe off of up to 50 coats won hands down, but certainl not efficient, contains Tueanal that causes Cancer to Californians. I did finish a table top with Varathane and left wet glasses and abuses on it for years in my shop and it never did ring or fade and it was applied right out of the can with a brush wiped and applied as an in the wood application...Used as a filler and finish takes forever..

Only thing I really was concerned about is how it lasted over a long period of time, like an Idaho camp rifle and loaner would survive.

This sort of thing only happens after the boredome of retirement, how accurate it is I have no idea..but makes me feel it was worth the test..

I also like the"sweedgie" method of application towards the end of finishing..Not sure I got the spelling correct on that..It the t-shirt material cut in 4" squares with a cotton or foam ball and tied off. dip it in squeeze it out and apply, it gives the thinnest most even coat of finish I can imagine and you can quickly wipe it dryish and hang it up for 12 hours. It works well with any finish..mostly used for piano finishing apparantly..

That sounds like a severe test, but certainly speaks to durability. I wouldn't be up to applying 50 coats of finish, retired or not. I'll check out the Brownells finish you mentioned for some future project. Almost done with this one, ended upstanding the shine off of it as Heym SR20 suggested, but then using a satin spar varnish (same brand as the semi-gloss I started with). It's about what I had in mind. Not the easiest finish to apply, but it's turning out well.

I did a spar varnish for filler using the sweedgie cotton ball wrapped in t shirt materal, then came on top of that, after sanding back down to wood and checking the fill, I used the sweedgie balls to apply VArathane, and it was an awesome finish, but it would not work for a gunsmith with a customer in a hurry...It took months not weeks, but was super nice and has lasted for many years now..

All in all I like brownells Gun-Save-R oil modified Urathane I think..Its easy and fast and last real well and seems to link with modified Linseed or tung Oil with dryers for beauty but polish off the top layer and finish to get that old world finish,but still durable.. I call it my bruja finish and I liked that finish, but in reality Brownells Gun-save-R "satin" is just as good!!