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Finishing - Polyurethane Finish

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When using polyurethane varnish over silk wraps, is it best to add a fresh coat every few hours before it can completely dry?  I let the first coat dry and that left the shiny little reflections that I have seen others talk about.  (Hal Manas)

The reflections you see are the result of small uneven places in the finish. They probably originate in slight unevenness or overwraps in the thread. I think that most of us use several coats of varnish with sanding in between. Building a thick coat quickly results in sort of blob like wraps (sort of like a high build epoxy finish). Also it just replicates the unevenness. After you coat wait about a day and very lightly sand with 400 or 600 wet or dry. re coat and repeat several times. Once you have built up a fairly thick surface wait until the wraps are  quite dry  (usually 3-4 days) and sand to remove the uneven spots, taking care not to cut into the threads.  I use # 600 paper glued to Popsicle sticks or fine emery boards to smooth the surface. A couple more coats and you are done. Maurer and Elser's book has a good description of how to do this.  (Doug Easton)

Good advice on the need to sand the built-up coats of varnish (or poly).  It's important to fill all the thread grooves, but one needs to get to that point, and a little beyond, before any sanding is done.  Prior to this stage, it is far too easy to break through the finish and raise "fuzzies" in the thread.  I find that at least five or six coats are needed before I can safely do my first sanding.  Then I proceed with three more coats and do a final sanding before finishing the entire rod.

As to the "shimmers," I do not believe they are caused by uneven places in the varnish.  And an overwrap will jump right off the rod at you -- it's a very different kind of problem than an area of shimmering.  The shimmers (when the condition rears its ugly head) will be visible immediately when the first coat dries -- and even more apparent as successive coats add translucency.

Someone on this list suggested  that  shimmers  are  caused  by micro-bubbles that remain within and amongst the fibers of the silk.  I don't know if this is the  explanation or not, but I think it may well be.  These bubbles need to be driven out (or displaced), but merely thinning or heating a conventional varnish does not seem to lick the problem

I think the trick is to use some product that will penetrate the silk fibers in a more uniform and complete manner than would a conventional varnish (or poly).  In this, I have had excellent results with Birchwood Casey's "Tru-Oil,"  but there may be other products that would work as well.  I apply my first two coats with the Tru-Oil, wait two days, and follow up with successive coats of poly.   So far, I have had no more difficulties with shimmers.  (Bill Harms)

This might be  heresy of wrap coating but it works for me.  This would not solve shimmers problem but for quicker flattening the grooves or mountain of threads.

While I was following the ordinal varnishing procedure on silk threads, it is needed for me to coat so many times until the thread's mountain is gone.  When the mountain goes somewhere and the surface gets flat (though smother up and down are still there), the varnish is already got fat (thick).

Sometimes I tried to sand, file the mountain off between coats.  Because I am a man who hate to wait, I did sand, file the varnish before it become hard and completely dried. ( It is an ordinary good method to sand, file after the varnish get hard though it still has a risk of scratching the threads and the scratch of threads becomes hairs of varnish.)  In those cases, rather thick layer of varnish was taken out and it made a surface more terrible up and down and it is very ugly.  But I can't wait until the varnish is completely dried between each coats.

Then, I considered.  While the varnish is still wet and soft, isn't there any good idea to flatten the surface?  When constructing a road, they are rolling over the cement while the cement is still soft.  When my wife is making up, she is filling the groove on skin with something creamy and powder on it.   Soft soil can become a good statue. etc., etc..

I made a very thin knife (1/4" width very thin)  from a piece of Tonkin with remaining enamel on it.  After epoxy (flex coat) is applied onto silk threads and it is dried up, I brush polyurethane on the threads.   Poly is not thinned.

It is important to wait, this time, until the epoxy is completely dried up.  Around when the polyurethane gets half dried, it is still sticky and pasty.  While poly is pasty, I take a thin knife of Tonkin and run it on the mountain of threads with enamel side of knife (as it is smooth).  It will move the varnish on the mountain and fill it into the groove and the surface becomes a little flat.  I repeat this twice or three times while varnish is still wet.  I can do it twice or three times a day as I do it while poly is still wet. So my varnishing is not really a finishing, but just putting the varnish on the object and wait until it becomes pasty.

When the surface is flattened enough to be ready to finish, I have to make the varnish on wraps completely dried to further flatten the surface by sanding, filing.  After it, I apply rather thinner varnish on it and get flat and shiny finish.

Care is needed to process the right edge of the threads.  It is easy to be pop up or scratched.  I named the scratch of silk thread as "hairs of poly".  When I find "hairs of poly" coming out of surface, I shave it with very razor sharp knife, or fire out.  But pasty poly will make it fall and past it on the surface.

It might be also a way by rolling over the soft poly with a round stock bar of NS or Brass to flatten the surface.  (Max Satoh)

The solution to the problem that works for me now is to start with Tru-Oil for the first coat, thinned Tru-Oil for the second coat and then whatever you use for the remainder of coats.  This has allowed me to put on wraps that make me proud.  (Hal Manas)