Learning from the Past - Spray Gun Tips and Needles

My last restoration was of a European carriage of lovely design, but extremely rustic materials!
Knotty Scotch Pine to be exact, complete with sappy, barky knots!

I spent countless hours leveling ripply grain and other defects.

And then I primed.  I used a sanding primer that wasn't necessarily noted for high-build properties, so it was probably a medium-solids primer.  I primed and sanded and primed and sanded, until I had a glass-like surface to paint.

And then, when I went in to top coat, all the grain, all the knots, all the defects had "photographed through", leaving me with nearly as rustic a surface as before I began to prime!

What happened, I never knew, even though I may have had the information right in front of me.

Re-reading an article I had printed out on setting up an HVLP gun, I realize that the "shrinking" of the prime coat may have been due to incorrect selection of the needle and tip for the paint gun.  The article likens the particles of paint sprayed as either like a jar of BBs or a jar of marbles.  The space in between is solvent.  The smaller spray tip and needle will produce a film of "BBs" with very little solvent in between the particles.  The larger spray tip and needle will produce marbles with lots of solvent in between.  With the larger spray tip, as the excess solvent evaporates, the film of material actually "shrinks".  After curing, a coat of medium solids primer sprayed with a 1.3mm tip and needle may be considerably thicker than the same paint shot on with a 1.7mm tip.

The primer in the above photo was sprayed with a 1.5mm tip - I'm guessing that I would have less "shrinkage" of the paint and primer, and less "dieback" of the gloss, had I tried spraying with the finest tip that I could have gotten away with.  For the above paint, I'm guessing I could have used a 1.3mm tip.

For the current project, I am using a gun with a 1.7mm tip, a large expense that I could have done without.  I am spraying high-solids, "high build" primer, but I am guessing that I could use a 1.5mm tip.  This gun doesn't have one.

Ah, but this one does!  And a 1.3mm, a 1.4mm and a 1.7mm.
The 1.5mm happens to be loaded in the gun.  This is my top coat gun.

This afternoon I should be able to spray the fourth coat of primer on the wheels and I will be trying this gun out instead of the big "hoser".

The article that describes these details can be found here.









UPDATE  

I did, in fact, get the fourth coat on the wheels this evening, and with the new gun and the 1.5mm tip, things REALLY began to happen!  I could see instantly that the paint was much more finely atomized.  Then I began fooling with the amount of paint the gun was delivering - increasing it - and I saw grain completely disappear under the new coat. This brought out the rabid restorer in me!  The wheels are starting to look so beautiful that I am determined to do a much better job of priming than I was prepared to settle for.  During the coming week, I will probably use some 3M Acryl-Green spot putty in deeper grain, and finish up with a couple more coats of primer, totalling six, before wet sanding.  I am really happy!