"AHilton" wrote: (clip) Polyurethane don't mix. (clip)
^^^^^^^^^^^


Firstly, that "(clip)" should have been "Lacquer and Polyurethane don't
mix". Let's not give wrong impressions by quoting out of context.
Polyurethanes mix quite well and need to be mixed quite often as well.


I recall from the days when I was a paint dealer, that polyurethane had a
problem with adhesion to itself, because it dries so hard. So, if the
recoating was done more than a day later, sanding was especially
important.
Urethanes also dislike sealers containing stearates (soaps). But, once a
lacquer film is dry, I do not recall any special reason why urethane would
not adhere (to a properly sanded surface.)


I don't think the adherence problem with polyurethanes has to do with actual
hardness. Sanding (or, more precisely, having a surface where the poly can
"grab" onto) is important regardless. Unfortunately, it's not easy to
smoothly sand a wet poly surface. g You're not gaining anything by
recoating poly until it's completely dry because that layer isn't going to
bind to the underlayer chemically. A "softer" poly mixture doesn't bind any
better than a "harder" mixture from what I've read, heard and experienced
myself.

Yep, those stearates are slick. That's kind of the point of being a
"Sealer"... it's easier to sand that way. The Urethanes, simply because
they bind mechanically, don't get much of a grab on a slick surface. Simply
sanding the surface, in this case, properly and smoothly isn't going to do
much to help. You've still got those slick stearates laying in there. Now,
this isn't to say that you can't make your own or use a "Sealer" just by
using a thinned version of the finish you are putting on top. Of course,
this isn't really a "Sanding Sealer" either.


There is often confusion due to the fact that lacquers act like paint
remover over oil based paints and varnishes. In that sense, I can see
that
lacquer and urethane do not mix. Even there, however, if very thin coats
of
lacquer are misted on, allowing the film to build up while releasing the
solvent, it is possible to create a barrier which will allow the lacquer
to
adhere and not damage the undercoat.


True. But we're talking about Polyurethane here. Actually, the original
poster said he was using lacquer sealer underneath the poly. I had
mentioned the reverse situation and mentioned that "Lacquer binds chemically
(with other finishes/paints/itself/etc). " as you mentioned above. Again,
as you said above, I've seen acceptable results with very very very light
lacquer applications over *some* other finishes. The lacquer thinner will
dissolve the underlying finish and bind fairly well. However, in the case
of Polyurethane, that's just not the case. They just don't mix and, in the
case of lacquer-over-poly, there's no means to bind the two together
acceptably. Extremely little chemical or mechanical bonding going on there.


Does any of this jibe with your understanding? Have things changed since
I
learned about it, or is my memory slipping.

MANY things have changed ... but mainly in the water-based finishing world.
Not much is different otherwise. Just new techniques and, perhaps, better
understanding of what's going on chemically. If anything, there's been a
lot more debunking of myths and "creative marketing" in the past 10 years.
So much in the finishing world has been twisted and confused due to
marketing and slack use of terms ("sealer" being a huge one). I hate to
perpetuate these poor use of terms but not many lurkers would know what was
talked about otherwise. I try to use quotes around things to suggest a
common (if incorrect) use of that word. Anyway, the tools and techniques to
actually discover what's going on in those finishes has become relatively
easy and cheap lately and this has lead to the debunking I think.

Frankly, I mix/prepare almost all of the finishes I use on my woodworking
and woodturning pieces. I know exactly what's in there and how old it is.
I'm not a "one finish" type of person either. I use all kinds of finishes
for all kinds of things. I can't proclaim myself an expert in the world of
finishing but I've done my fair share of testing, using and royally screwing
up that last coat of finish and starting all over! g


- Andrew