some qualifications for the claim
Response To:
Grip Strength ()

Bill Tindall, E.Tn.
varnishes like polyurethane that don't re-dissolve the prior application (not ignoring the wet-on-wet method) have to be sanded for the next coat to get a good grip on the existing film.

One layer will(or won't) bond to the next depending on the curing chemistry, adhesion between layers and any mechanical interlocking due to roughness. I do not have data to support this claim but I would expect there to be sufficient unsaturation remaining in a cured varnish film formulated from "oils" to chemically bond to a layer of new finish added any time later. From smell one can tell curing chemistry is going on for days to weeks in a varnish finish. So over this time scale chemical bonding will occur.

Neither chemical bonding not mechanical bonding is necessary to get layers to stick if the surfaces stick by adhesion.

The problem with sticking to an aged surface is this aged surface is likely chemically different from what it was when freshly applied. I can think of a lot of reasons new varnish might not stick to a 5 year old finish that lived near the kitchen.

Personally I have never experienced a finishing failure due to layers of finish not bonding.

© 1998 - 2017 by Ellis Walentine. All rights reserved.
No parts of this web site may be reproduced in any form or by
any means without the written permission of the publisher.