Why Should We Thin Poly Before Rubbing the Finish?

I am reading info on rubbing finish. There is one thing that I don't quite understand. The instruction indicates that we should thin the finish, let it hardened, and then rub the finish. This instruction may be good for other finishes; but I am wondering whether this is practical for poly. According to the instruction, the poly finish must be thick; otherwise, when I rub the finish, I may run the risk of rubbing through the top coat of the poly and revealing the line between the top coat and the second last coat. But if I thin the poly for the top coat, I will get a thin coat and I will have a high risk of rubbing through the top coat. Something is missing here, right?
Is the idea of "thinning the last coat" more appropriate for other finishes that one coat can melt with the next coat, and multiple coats can become one thick layer?
Thanks for any info in advance.
Jay Chan
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Thinning the last coat is different that building up multiple layers for rubbing out. thinning the last coat (or every coat for that matter) helps the finish to smooth out and reduces the size of brush marks, thus making the entire polishing procedure go faster and easier.
Manufacturers are making finishes thicker to comply with VOC regulations as there is less oil or carrier to flash off to the atmosphere. By adding a little thinner back in, you can get a better finish, even though more coats may be needed.
Let that poly cure for two weeks before rubbing it out.
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Polyurethane was never meant to be a fine furniture finish, subject to the finishing techniques and applications that shellacs, lacquers and other build finishes require.
As you know, poly is a film finish, and will stack onto itself, coat upon coat. It will certainly adhere to the previous coat but it will never become one monolithic layer.
The obvious advantage to a build finish is that you never burn through a layer of finish, so you can "rub" or polish to your heart's content. That is why it is used today and has been used for a few hundred years as such.
But some here report great success with rubbing out the final coat of their poly finishes. I have only hit a poly finish once with rouge (a desk top) but never tried the full treatment of cutting down the surface to a mirror reflection. Never had the confidence it would work.
I would be interested to see what you try.
Robert
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With the exception of one product that I know of, most poly is not meant to be rubbed out. According to Jeff Jewitt, rubbing poly often produces a none uniform sheen. And yes, applying a thin coat of a layered product like poly before rubbing out the finish does not make sense.
Cheers, JG .

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Edwin Pawlowski wrote:

Hi, Ed. I am just thinking out loud here... just speculating. I thin my poly about 10% when I spray, and about 20% or so when I pad or foam brush it on (depending on temp, etc.). I do that with every coat. I learned the hard way that putting coat after coat on an uneven surface will cause a lot of heartburn. Thinning does all the good stuff you mentioned. But your post got me thinking...
Since the "rubbing out" of the last coat is the finish coat and you need enough material to cut down the highspots (brush or lap marks), wouldn't it make since to put on several thin coats all carefully applied to maxiumum smoothness, and one last layer of pretty heavy to that you would have plenty of film thickness to work with to level out and polish? I have no experience in rubbing out poly, so as I said, this is just wondering aloud. I would like to hear your thoughts, though.

AMEN! I was using Defthane for a while and their industrial coatings rep told me that it was "green cured" at 13 days, and fully cured in about 25 - 35, depending on the conditions of application. When I use poly I don't even sand for final finish coat for at least 10 days or so.
Robert
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I have limited experience rubbing out poly, but you can get a truly excellent finish if you take the time. I did one a few months ago and was astounded by the results. Not plastic looking like poly usually is.
I think you are right, but we should clarify something.
Thick = large dimension Thick = high viscosity
Thin = small dimension Thin = watery
There is a difference between putting down a heavy coat versus putting down a coat of thick (high viscosity?) material. The last coat should be flowed on, not dragged out thin, for good results. Your 10% sounds about right. So, a thick coat of thin poly sounds about right so it will smooth out for easier sanding.
Make sense?
--
Ed
http://pages.cthome.net/edhome/



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Edwin Pawlowski wrote:
SNIP

Yes, it does. I had to read it a couple of times, but I see what you are saying and it makes sense. The flow out of the finsih on the piece would be the key.
Although this thread is about poly, I think lacquer (although not as sturdy a finish) would be much easier. For lacquer I build a coat as recommended by my supplier to about 9 mil (I know... I thought he was kidding the first time) and then plan on cutting it back to abou 6 mil when finished. I sounds like a lot, but the three mil I take off is only equal to the thickness of a dollar bill. It requires smooth application on all coats to get to 9 mil.
Do you know how much material (thickness) you are cutting off when polish out the poly? Do you remember what brand you used and how many coats you applied before rubbing out? One last one, what did you use to rub out the finish - hand powered or machine powered, and what grit/polish did you use?
I appreciate the response. I would love to set up an experiment to try this and have a left over quart of poly to try it with. This is how I learn. It would kill my associates/competitors to find out I had rubbed out a poly finish, even if it didn't look french polished.
Thanks -
Robert
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snipped-for-privacy@hotmail.com wrote:

Possibly yes, but with very particular meanings of "rub" and "thin".
In general, don't rub out polys. If you want to rub out a finish, use one (like shellac) that merges well between layers, not one that leaves distinct individual films.
If you do rub out poly, use pumice or rottenstone and nothing more. If you rub out enough to penetrate through a film it will never look right. Some people favour the plastic mesh abrasives instead, although I've not used them myself. All I ever do is to use rottenstone (not even 6/0 pumice) to slightly dull a poly finish and avoid the "plastic kit" look. I do this with gel poly when I want the hard-wearing and spillproof quality of a poly, but would rather it looked as if it were oiled.
If I want something to look like french polish, then I french polish it.
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snipped-for-privacy@hotmail.com wrote:

Thanks for all the helpful info that people have offerred here.
Seem like trying to rub-in poly is asking for troubles. Next time I will use other finish that is more suitable for rubbing-in.
Jay Chan
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