Turn Gravity Upside-Down

˙˙˙ǝɯos ǝpɐɯ ı 'puǝʞǝǝʍ ʇsɐd sıɥʇ
oops! sorry...
This past weekend, I made some trivial plywood-based cabinets for the inside of a closet in a spare bedroom that will someday (!?) become a reading/Foosball room.  I don't need the space for clothes, I need it for stacks of printed pictures (yeah, pre-digital, people printed photos!), CDs, hanging files, and a wireless printer.  I sprayed it with General Finishes PolyAcrylic water-based clearcoat.  But the reason for breaking the recent silence on my blog is to talk about how I sprayed it.

I have a gravity-feed conversion HVLP gun from Jeff Jewitt's finishing site.  A conversion HVLP gun uses compressed air as a propellant instead of high-volume turbine air.  It works great with my 20 gallon compressor.  "Gravity feed" implies gravity is somehow used to this gun's advantage.  The weight of the finish in the cup helps this low-pressure gun pull it down into the nozzle for atomization.  So you need to keep it pretty much upright.

So... how am I gonna finish these narrow shelves?!

Rather than use the gravity cup from the sprayer, I installed a 3M PPS cup.  You can see the system in this picture.  From top left clockwise, the rigid plastic cup, the disposable collapsible finish cup, the cap with built-in filter, and the black locking ring to hold it all together.  My crooked finger is pointing to an adapter that takes the place of the original gravity cup; this is selected for your gun.  While the attachment point is gun-specific, the top is how you attach the PPS cup.

Here you can see it all assembled on the gun with the original cup for reference.

The cup collapses during use to keep the fluids in the gun's intake, but we'll cover that in a minute. A nice benefit is that there are caps for the PPS cups so when you are done, pull the collapsed cup out and cap it.  The black cup is full of dyed PolyAcrylic from over a month ago and it's still just fine.

The lid has a built-in filter.  What's nice is that the caps are cheap in quantity so if the filter gets gunked up beyond reason, toss it.

Put the collapsible cup in the plastic cup holder, fill with finish, and press down the cap.

Lock the ensemble with the black locking ring and attach the gun.  The gun simply pushes on then a twist locks it.

The plastic cup has a hole in the bottom to allow air in for the collapsible cup.  Use that hole for your finger while voiding the cup of air.  To do this, press on the collapsible cup while pressing the trigger on the gun (connected to the compressed air).  The Venturi effect will help pull the air out (finger helps... my gun is very low volume; higher pressure guns can do this sans finger).  Quickly, the cup with collapse with no air at the top and finish will shoot out the nozzle.  Ah, that's the cool part...

Maybe I should have used black finish for this shot :)  With the air voided, I'm spraying upside-down.  You'll likely need to open the picture to see against the cabinet side.

Cooler still, it works in any orientation since the Venturi vacuum keeps the finish at the inlet.  Here I have the unit on the side spraying the sides and the shelf surface (at a slant).  Works very very well.

Putting down a gravity-feed gun is annoying; you need a stand for it and they seem built to give you the feeling that they'll drop their payload the second you walk away.  Since the vacuum stays for a very long time, I just set it down upside-down in an office wastebasket: it can't fall over and putting it down it a quick lowering by the air hose.  I left it like that for an hour between coats with a damp rag over the nozzle to keep finish from drying.

This is the collapsible cup after a lot of spraying.  To remove the cup, detach the air hose, flip the gun, and squeeze the trigger to let in air.  As soon as you do that, you can easily twist off the cup and cap it for spraying the next day (or month!)  Here I intentionally banged the cup to let in more air for demonstration.

This HVLP gun is excellent to use, easy to clean (all stainless steel), and with the PPS it...
¡ob ʇ,uɐɔ spǝǝɟ ʎʇıʌɐɹb sǝɔɐןd uı sʞɹoʍ

As an addendum, spraying those shelves on a slant worked very well.  One thing to make finishing those areas easier is to first spray the flat shelf on a slant first (slanted because you can't get the gun twisted upright) then spray the sides.  Due to the pretty extreme slant used for the shelf, you're gonna get overspray on the sides, but since you'll be spraying them seconds later, it isn't an issue.  You won't get much overspray when spraying the sides.