blueman wrote:
As a newbie to HVLP, I have encountered the following problem...
I am spraying Minwax (water-based) Polycrylic semi-gloss polyurethane.

Although the spray seems to atomize well, it seems that unless I put
on a fairly heavy coat that it deposits in discrete though very small
droplets resulting in a somewhat rough surface.
i.e. - the spray doesn't seem to coalesce into a uniform coat unless I
spray it on pretty thick. It's almost if some type of surface tension
is causing the spray to coalesce together rather than completely and
uniformly wetting the surface.

When I move the gun slower to let more finish deposit, I do get a
uniform sheen that completely 'wets' the surface (however this only
works when the surface is horizontal because otherwise it would run).

I purchased the baby brother to that gun (46719) for detail work. Almost
all of my painting is with automotive refinishes, and I don't do much at all
with the stuff you're working with, but I'll share some comments and
experiences in general.

First off - don't assume that you'll get sags or runs on vertical surfaces.
If your gun is set up right, the material is the proper viscosity, and your
technique is right, you should easily be able to spray vertical or
horizontal surfaces with that gun. If you have to put on what you feel is a
heavy coat to get a wet coverage, then you probably need to either thin your
material or step up to a larger tip. As well - be careful with HVLP about
cranking your input pressure up too much. More is not better with a gun
like this. HVLP does not atomize like the old guns we were used to and it
really takes some experimenting to get used to your gun.



- Is this normal or am I doing something wrong?

Of course - it's always the sprayer's fault - never the gun's fault...

- If so what are the likely causes and corrections?

Generally - too high input air pressure, or too small a tip, or too thick a
mixture.


Note: I am using the low cost but surprisingly well reviewed Harbor
Freight #66222 (purple) gun. My 8 gallon compressor seems to have
enough reserve to power the gun since it is able to cycle. on and off
and the pressure never drops below about 70psi.

The surface I am spraying is clean, well-sanded Birch plywood.

I have the inlet pressure set at about 30PSI.

Try cranking that back - a good bit. Try 15-20 psi.


I have made some (non-exhaustive) attempts to:
- Adjust the inlet pressure in the range of 20-40PSI

Too high for most HVLP guns.

- Adjust the fluid volume

Good adjustment - you'll be working with this adjustment a lot.

- Adjust the spray pattern (more round vs. more oval)

Don't use this to get the right amount of material down. Use this to get
the right coverage pattern. If you have to needle the thing down to a small
pattern in order to get a decent coverage, you need to go back and look at
viscosity, tip size, and pressure.

- Adjust the distance to the object (from maybe 8 to 16 inches)

16" is a mile away. Try moving in to 6-8"

- Adjust the speed at which I sweep across the object

Always. Different materials will require different application speeds.
Thicker equals slower.



While Minwax claims it hasn't tested and hence can't recommend
polycrylic for spraying, others have had good results with it. Also,
it seems to be of fairly high viscosity and hence seems to spray well.


Fairly high viscosity would not equate to spraying well. You might want to
add 25% more reducer than you've been using and observe the differences.

--

-Mike-