Poly question

[Tim Hanus]

Hi Guys and Gals,
I have been reading a good while now and have found tons of useful information. I have a question concerning polyurathane that I was asked this evening and did not know the answer to. What happens if you do not sand in between coats as the directions dictate? I understand that poly does some sort of chemical reaction thing and the sanding helps the bonding. Will it still bond without the sanding?
Thanks in advance,
Tim

[Steve Schoene]

If you apply a subsequent coat within a narrow window--cured enough to not be excessively disturbed by the solvents in the next coat, but at the same time still enough uncured that you can still get chemical cross-linking between the coats--then you would get a satisfactory bond. How long that window is depends on the particular varnish and upon temperature and humidity where it is being finished.

That said, the single part consumer grade varnishes like you might find at the big box stores or local paint stores don't provide the technical data to allow you to determine how long that recoat window is. The window is likely to be much longer for the superior non-polyurethane varnishes, but even with them you don't want to wait too long for overcoating without sanding.

And, polyurethane varnish has rather poor adhesion characteristics anyway. It needs all the help it can get. The sanding provides a mechanical "tooth" for the next label to grab to if it can't cross-link chemically because it has already cured too much.

If you have to use polyurethane varnish, then I'd really not try to guess at the window and let it cure overnight, sand, and then recoat. I'd even do that with brushed on non-poly varnishes. Besides varnish collects enough junk that sanding each coat makes the final rubbing out much easier, with less risk of cutting through.

[Howard Acheson]

Poly varnish has quite a bit lower adhesion strength compared to a non-poly varnish. The reason for scuff sanding between coats is to provide an adequate "tooth" for the next coat(s) to grip on to. As Steve said, if you overcoat within 4-8 hours, you can general get adequate adhesion without sanding. But determining the time is tricky. I use a fingernail test. As soon as the surface is no longer tacky but will show a fingernail impression easily, I will consider not sanding.

At the same time, the best looking finishes are those that are a flat as possible. Between coat sanding can flatten hills and valleys and reduce or eliminate brush marks producing a nicer looking finish.

[Tim Hanus]

Thanks guys, that is what I was thinking but wanted a consensus from other experiences.

[Jim Becker]

And a better long term solution is to use non-poly varnishes on your woodworking projects. Nicer looking (more clarity) and none (or substantially reduced) of the adhesion issues mentioned. You'll want to shop at a real paint/finish store, rather than the 'borg for that.