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Prometheus

On 11 May 2007 15:19:40 -0700, wrote:

I'm having a problem that is driving me nuts. I've been using qualalac
and it is cracking. I mean lots of cracks. When I first noticed it I
thought it was the wood but the cracks are in the finish. Moisture
content in the wood is not an issue as the pieces that this is
happening to are quite dry at about 6%. 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

I've never used qualalac (in fact, I'm not even sure if that is the
name of the laquer you're using or the wood!,) but I have used plenty
of laquer, and experienced the same problem from time to time.

There are three main things that have caused a problem with laquer for
me in the past, and one, all, or none of them may apply in your case.

The first two are related- excessive sunlight and/or heat when curing.
I had a project not only crack, but even develop bubbles after
laquering because I left it in front of a window on a hot, sunny day.
Sometimes it's tempting to rush it, but laquer doesn't like being
hurried (and it's really not that slow to cure in any case.) If
you're setting it near a window, try putting it in the shade to dry.

The third is something I doubt you're doing, but here it is. Layered
finishes- in my case, this happened when trying to exactly match an
old, semi-transparent stain for a really picky customer who *wanted*
the trim I was making for them to look old and (IMO) crappy. The only
way to match the color was to do the same thing the original finisher
did- to paint the stain on thick, and let it dry without wiping it off
so it would deaden the grain pattern. When I sprayed the laquer
over, it was able to slightly dissolve the stain that had cured on the
surface, and ended up cracking pretty badly. The solution ended up
being to paint the stain on, then follow that with a coat of shellac
to act as a barrier between the stain and the laquer, and then laquer
it. It looked like junk to me, but the customer was happy.

If there is something in the wood that is attacking the finish, you
may want to try the shellac trick to seal it before the final topcoat.
I used a 1/2 pound cut, and wiped it on with a clean rag. It will dry
within minutes, and you can proceed to the final finish. If you're
doing a really complex layered finish, I'd do this between each layer-
the reason for the shellac is that it is alcohol based, and does not
dissolve (or does not dissolve much) when it is covered by most
commerically availible finishes.

Though of course, I'm still fairly convinced that finishing is more
voodoo than science- the above is just what worked for me when I had
the same problems with laquer cracking.