Question
I
have a customer who has specifically requested a sprayed polyurethane
finish. I use a conventional Spartan spray gun, no HVLP or LVLP yet. The
finish is a bit too rich to atomize properly. What can I use to thin
the poly to get a good laying finish?
Forum Responses
(Finishing Forum)
From contributor C:
You
can use mineral spirits, naptha, lacquer thinner... assuming that you
have an oil base poly (not a waterborne). Naptha and mineral spirits are
safer choices, but I often use lacquer thinner and even combinations of
any of these products. Mineral spirits are the slowest drying and
lacquer is the fastest of these three. If you want something really
slow, you can use kerosene.
From contributor M:
Be very
careful if you decide to use kerosene, as it is very slow drying. Only
add a few drops. As they used to say, a little dab will do you. First
try the mineral spirits, and then check the dry time. Adjustments can be
made depending on the size of the pieces you're working on.
From contributor D:
This
is really a very bad idea. The overspray will be wet on everything.
Spraying polyurethane is 500 times worse than spraying precat lacquer or
conversion varnish, as the stuff takes forever to dry. This just isn't a
problem on the article being sprayed, it's more of a problem with
respect to the spraying area. You really don't want to do this, trust
me.
Thinner PU là gì
From contributor M:
A few drops of kerosene would have
very little effect not only on the overspray, but very little effect on
the flow out and the dry time, although it would change them a little.
This is also true with adding a few drops of this distillate to an oil
glaze; if you increase the kerosene in either the polyurethane or the
oil glaze, it will take forever to dry. It's the amount you add that
makes the difference.
From contributor T:
Be aware that you
will have a sticky mess all over your exhaust ducts and blower fan that
will eventually be a pain to scrape off. Filters don't stop all of it.
Fast drying finishes just blow away; slow drying finishes stick and
become tough. I would talk them into using a waterbased urethane finish
like Fuhr 255 if it were my client. Look into a w.b. polyurethane. Tell
them Varathane is for brushing.
From contributor W:
I sprayed varnish.... once. It should be brushed or thinned way down and wiped. I wonder why anyone would want to slow it down?
From contributor M:
There
are many companies today that spray poly and have been doing it for
years. Like any other coating, there is a learning curve, and the proper
solvent reduction and the gun adjustments are two of the things that
need to be worked out. Normally, poly is slow drying, and lacquer
thinners are added to speed it up, but there are many different
formulas. Just as most other coatings have additives, so does
polyurethane, and lacquer thinners and kerosene do work in poly in
moderation.
From contributor C:
It is true that the overspray
from slower drying products like poly is much more troublesome than that
from the catalyzed finishes and lacquers (which is usually dry before
it lands on anything). Since I have switched to HVLP, I have not found
this to be very annoying, but before, when I used a standard spray gun,
it used to coat everything in the shop. I use it rarely now and in
moderate amounts and have few problems. The overspray is reduced by
about 90 percent with my HVLP vs a standard gun. You may want to pick a
nice day and spray this project outside and then bring the pieces in for
drying. Watch out for bugs, though... I just hate it when a giant fly
goes swimming in a freshly coated door or a huge herd of gnats descends
like locusts to devour my livelihood.
From contributor R:
Overspray
is the least of your worries. With an open time (before dust free) of
an hour or more, any dust, gnats, flies, sawdust, stupid carpenters,
etc. can ruin your finish. I don't know about you, but I work in a
cabinet shop and dust is everywhere. Even though I have a closed off
area with a pressurized room, I still wouldn't want to use poly on
cabinets. The other thing is problems trying to glaze or shade. I would
tell the client that it's a bad idea. Do some samples with a product
you're comfortable with and show them to the client. They usually will
respect your opinion as a professional.
From contributor W:
Gotta
get up early to keep up with contributor M - there certainly are a
bunch of poly(somethings) out there and most are easy to spray. Because
the "finish was a bit too rich," I have assumed it is a non-catalyzed
oil based finish intended to be brushed (so, naptha, MS, etc. are the
solvents to use). If it is, it polymerizes by oxidation - a sloooow
process, not by solvent evaporation - a faster process, and not by a
catalyzed reaction - a very fast process. So it's going to stay wet for a
long time (as has been stated). Now we're spraying this from a
conventional gun (transfer efficiency 35% or less). That means that even
with your gun perfectly dialed in, 65% of this wet finish spray is
going to go into the air as overspray and it's going to come down
somewhere and it's going to be wet when it comes down and stick to
whatever it comes down on. So even if you spray it in the garage on a
windless day and have eliminated all possible airborne contaminants, it
would be a good idea to cover the bikes and lawnmower.
Aside from
that sage advice, I would recommend that you get a waterbased poly
(acrylic or urethane or both) and rent a little turbine HVLP to lay it
down. You'll need half as much finish, it'll dry before the bugs or dust
can get to it, your recoat time will be much shorter, and you'll be
able to sweep up any over spray. Or if you can handle it, a CV would be
even better.
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