On May 12, 2:03 am, "
wrote:
On May 11, 5:19 pm, wrote:

SNIP

I'm mixing the lacquer with
lacquer thinner 50% as per the directions. I'm using a small hvlp sprayer.
I used to use nitrocellulose and never had a problem.
I can't find it anymore or I would still be using it.
Has anyone had this problem with qualalack or know what is causing
this?
TIA, Bob

Hi, Bob. First, really nice work on that website. I think that
spalted maple bowl with the square edges is really pretty. Well
designed, too.

The following may be more than you are interested in, but it might
help others too. Me, I like to have as much info as possible, so here
goes.

I'll try to hit the high points of your post with some ideas
So that you won't think I'm just getting started in finishing, look at
rec.woodworking under this same name, and you can see that I have
written extensively on finishing (especially lacquers and
urethanes), as well as on Woodweb. As a remodeler, I spray about 50+
gallons of lacquer on a heavy year (you read that right), and a little
less than half on a lighter one. Along with industrial enamels and
urethane coatings, I have had to break down and study, learn, practice
and "trial and error" my way to finishing that I like.

I started finishing for myself many years ago when the people I had
subbed the cabinet finishing to didn't do a good job and couldn't find
their way to work except on Friday.

Anyway - there are a lot of different things that go on with lacquer
and solvents. You are using a nitrocellulose lacquer, so you can use
just about any brand of quality lacquer thinner that is out there on
the market. Crown, Strip Ease, Sunnyvale, etc. are NOT quality
thinners and who knows what they have in them. Not even the
manufacturers know exactly. Many of these low end lacquer thinners
are actually made from recycled materials, and are almost unusable
with some finishes.

Buy a good quality branded lacquer thinner like Sherwyn Williams,
Benjamin Moore, Startex, etc. There are others, and these are paint
store items. cracking lacquer is difficult to do, so you may just
have bad thinner.

Try thinning much less. The instructions on the can are guidelines,
and they are a place to start when you are unfamiliar with the
product. As a high VOC evaporative finish, lacquer will crack if it
dried unevenly. Under certain conditions, say a warm dry area or an
area with constant airflow or movement, the lacquer will begin to
skin over. However, it won't look that way to the eye, and you can't
see it unless you run your finger through it.

The finish cracks as the solvent attempts to outgas through the
partially polymerized finish. The solvent looks for the weak spots in
the "skin" and opens them up, pushing out the gasses through the
cracks. A large amount of thinner (are you sure they said 50%??) will
cause this to happen.

There could be contaminates on your wood. Stearated paper is
notorious for screwing up good finishing. Many buy the cheapest
sandpaper they can find, and overuse it. The lubrication (stearating
compound - commonly known as a variant of soap) will come off the
paper with heat, which of course is generated by sanding. This
lubrication will actually melt into the wood and cause all manner of
problems from patchy cracking to fisheyes.

I see that you do segmented work, and sanding down the different kinds
of oily colored hardwoods as well as the resins in the glues will
cause will generate a lot of heat and a lot of contaminates.

Try this: before finishing, clean the piece well with a rag wetted
(not just damp) with lacquer thinner. Really scrub it. You won't
raise the grain, and it does wonders for cleaning and will pick almost
all the stearate up.

Thin less. 50% is just too much. You did not say if you were using
an HVLP conversion gun (the type used with an air compressor) or a
true HVLP, the type powered with a turbine. But in this instance, I
don't think the problem is in the gun, as gun/pressure/delivery
problems are all about the same. I have both types, and in moderate
weather the thinning protocols for both types of gun are the same.
Start thinning at about 25%, and see how that does.

Put the right amount of sealer on your piece. Normally, the
manufacturers want about 3 to 4 mil minimum on a coat (some more) of
evaporative finish, and it will dry to about 1 mil. It is important
that you find the sweet spot for your finish, and that takes
practice. (To run down how to make your test bed and records to
establish the characteristics of each finish with your individual
equipment, go to the rec. and search there).

Thinner than 3 mil will NOT give you a good finish unless you are up
in the 5th-6th coat of material. For some build finishes a thinner
coat makes a good topper, but we aren't talking about that here. The
rule of thumb is that if you are laying down a thin even coat that is
drying shiny, quickly and evenly as you are finishing, then you are
not putting on enough material.

If you think your piece is contaminated with oils, stearates, or
anything else you might have gotten on it when you handled it, then
uses shellac. You quickly learn in fine finishing though, that
shellac will dissolve or blur those gorgeous spalt lines in many
woods. (THAT was an expensive lesson...) I would only use shellac as
a primer if I knew there were problems with adhesion.

Lacquer is its own best primer. Nothing sticks to lacquer like
itself. Why? Since lacquer is a build finish, properly applied coats
will melt into each other making a monolithic finish. Do not mix any
other coatings in between layers of lacquer for any reason as you
defeat the reason it is used, and that is the ability to build a
thick coating rapidly. If you put anything in between the coats, then
you will have a film finish, or layered. This can easily lead to
witness lines, poor adhesion, etc.

It is easy to have problems with the safest and most forgiving of all
finishes, shellac. For example: Put a coat of shellac on wood for
primer, sand and seal. Put a coat of shellac on top of that. But
wait.. did you thin the shellac to spray it or brush it easier? If
you did, you probably used alcohol from HD, Lowes's or the lumberyard,
and it is only around 96 - 97% pure.

Why is that important? The other three percent is water, and that is
the bugger to get out. This cheap alcohol is distilled from a number
of products, again sometime of unknown origin. However, if you go to
the paint store and buy anhydrous alcohol (99.99% pure) you are good
to go on your thinner/solvent. No water introduced into the shellac.
Sadly, this doesn't show up right away, as water is easily miscible
with alcohol, so you may have to wait for the shellac to cure out
completely before you see the problems.

As far as nitrocellulose lacquer availability, go anywhere they sell
lacquer. If it isn't pre-cat, post cat, or CAB (non yellowing white
with acrylic enhancement) and clearly marked as such, it will be
nitrocellulose. Most of today's lacquers are pretty good quality and
behave well under most conditions, so you should be good with
something off the shelf, nothing from a specialty shop needed.

The common brands are Deft (old reliable), Old Masters (my favorite
off the shelf brand), and there are a few others. Look for something
in the upper teens to lower-mid twenties of solids in the ingredients
when buying.

Good Luck! I hope you let us know how this all turned out.

Robert

Thanks Robert, A lot of good info in your post. The directions on the
can do in fact state a 1:1 mixture. Thinking that might be the problem
I cut back to 3/4:1 with the same result. I would guess the thinner
I'm using is ok as I've used the same thinner with success in the
past. With both qualalack and parks nitrocellulose.
The can suggests a 35-50# setting. Being an inexpensive sprayer I has
no idea what it is putting out.
My question to you is could the air mixture in an hvlp system have
anything to do with a poor (cracking) finish?
While I've used the sprayer for a few years my experience is limited.
Up until now its been point and pull the trigger.
My guess would be to turn down the air while still maintaining an even
flow.
Thanks, Bob