How many coats of Oxford USL? - created 02-06-2009

Ryan, Michael - 02/06/2009.17:45:42
Never waste the challenge of a big mistake

I am finishing my second acoustic SS OM model.

I am using Oxford USL water based lacquer and an asturo HVLP gun 25 psi at the inlet and about 2-3 complete turn from full open regarding material deilivered.

I have 6-7 coats on the top and 8 coats on the back, sides, neck. Last time I used about 11-12 coats back, sides, neck and 7 on the top to not dampen sound too much.

What are others preference, or experience.

I thought most people used about 12 coats on the back and sides and maybe 10 on the neck but I don't know.

I will wet sand and buff after a week of cure.

Thanks in advance.

I got a film thickness of around .005 after 13 coats, and about .008 after 19. I went a bit thick because I was afraid of sand through when leveling. No idea what my finished film thickness is after leveling and buffing.

I'd be worried about leveling and buffing that quickly though. I waited 4 or 5 weeks (had a very busy week 3 weeks after spraying) and the finish was plenty hard. I've read that 3 weeks is a good wait time. Perhaps spray some test boards and let us know what works after 1 & 2 weeks.

I've been using USL since it was PSL. I usually use nine coats on the body, heel, and peghead. I finish the shaft with several (six or more) coats of EM8000. When I've measured, my USL thickness is around .004" or .005", less after leveling and buffing. I'm using an Asturo ECO/SX gun with 35 pounds pressure when the trigger is open. I wait a week or two before I level and polish. The ECO/SX gun really improved my process. I can usually level quickly by hand (with a block) using either Abralon pads or P800 paper. Often, a pass with the P800 paper is sufficient. I level the neck shaft and then polish it with 0000 steel wool leaving it slightly less than glossy. The steel wool comes out only when I know I'm not going to be applying any more finish. It doesn't get along well with wet waterbase finish.

You said you have 35# with the trigger depressed. I can't get that with my system. It is down around 15# and I have a 4.5 hp porter cable compressor.

If I use above #35 static pressure I tend to have a very hard time controlling volume of finish sprayed - usually too much.

I am still learning. I think I'll go maybe one more coat on the top and 2-3 on the body and I am not sure about the neck.

I have the same Asturo Gun and do a little better with a little thicker water base finish like general finishes wat-r-base poly.

thanks

MBR

It's weird to me that you can't get the pressure. I have a cheap Devillbiss 3 hp (nominal--it's really a lot less) 12 gallon compressor. When I first got the gun, I started spraying with a much lower pressure and got a lot of orange peel. After consultation with Jeff at Target, I raised the pressure and started getting excellent results. You're not using one of those narrow coiled hoses are you? The gun has an air and a liquid volume control. I'm using the 1.3 mm tip. I managed to get everything dialed in so that it atomizes well and sprays a good wet coat with no runs. I never could get that with my old, cheap gun. I hold the gun 4 to 6 inches from the surface as I spray.

I don't have trouble with the 35 lb without the trigger pulled. I was recommended by target to get about 18-22 with the trigger pulled for the usl lacquer. I don't usually use more than 40-50# inlet from the compressor. I was told shouldn't go higher than 50-60 psi and reduce that to around 30-35 at the gun so I have 2 pressure valves inline so I don't have to be adjusting it every time I change from regular compressor use to HVLP gun.

I will fiddle with it again. I have the hardest problem with the curve of the om sides keeping the gun oriented at right angle to the surface all the time . I like the cone tip for the neck that came with the kit. I use the 1.4 mm tip that came with the master asturo kit.

I hang the guitar vertically when I spray. At the waist, I sweep across with the gun vertical. It works. What you get without the trigger pulled doesn't matter. What counts is the pressure when everything is flowing. Maybe the inline pressure valves are keeping yuo from getting the air volume at the gun?

About 9 coats is the minimum i am comfortable spraying with USL, more coats if you are concerned about rubbing through. It shrinks back a lot, and doesnt seem to get that nasty plastic film look until you start really piling on coats.

The longer you wait before buffing, the less shrinky glue lines wil show up. Two weeks is about minimum, three or four optimum. I know that they say 72 hours to full cure, but dont believe them, wait longer and it will sand and buff easier and most importantly, wont shrink back into the glue joints as much.

Here's a good (actually very good) discussion on the Australia/New Zealand luthiers forum on spraying and spray guns http://www.anzlf.com/viewtopic.php?t=259

Also here's a good detail on gun set up by Jeff Weiss specifically on the Oxford/Color tone WB. http://www.mimf.com/library/spraying_colortone.htm

These two threads really helped me dial my gun in right and I feel much more confident spraying after absorbing the details in both threads.