Could use some help/advice spraying shellac
I have a project where I’m using shellac as the finish. I plan to rub it out to gloss. I started out spraying 1.7 lb cut shellac, but I was getting some orange peel, so I cut it to 1.5 to try to get rid of the orange peel. With the lighter cut, I was struggling to keep the surface wet, and when I did get it to wet out by going slower and getting closer, I started getting little bubbles forming as the finish dries (solvent pop). Trying to get rid of the solvent pop, I lowered the pressure, increased my shellac cut to a 2# cut and tried using a 1.3 tip instead of the 1.2 tip I was using until now. It seemed like I was getting a LOT of material on the workpiece; so much so that it was pooling a bit at the downhill end and I still had the bubbles. So, I went back to my 1.2 tip, added 5% retarder, left the pressure down and stuck with the 2# cut. This produced my best result yet, but still not perfect. The bubbles are making it to the surface, but drying as little bumps. I think they’ll easily sand out, but I’d still like to refine my knowledge to produce the perfect surface. Any thoughts on where to go from here?
Here are some equipment and environmental factors if they’re of value
Shop temp is 75 degrees 60% rh
I’m using a Devilbiss Plus gravity gun with 1.2mm tip.
Any guidance is very much appreciated
Replies
Why use shellac?
Looks good; comes in many varieties; less health hazardous than nitro and if you mess it all up, you just wipe it all off with an alcohol soaked rag.
95% of the time I apply shellac with a spray gun. It's usually a fairly straightforward process. One suggestion I have is to spray a light first coat, particularly on coarse textured wood. This sparing first coat has the role of sealing the grain by coating all of the fibres and following the profile of the open wood texture. This prevents building up a thicker layer of polish in the deeper recesses of the coarse grain which is where air trapped within the crevices can push back forming a bubble in those thicker wet polish films. Sometimes it's a good idea to do a couple of light coats like this prior to moving on to applying thicker layers-- sanding lightly in between these first coats with about 400 grit paper may also be useful. This technique is one I use for other film forming finishes, so it's not unique to applying shellac, and nor do I reserve it only for coarse textured wood-- it can be useful on smooth wood too.
I also tend to reduce the air pressure slightly for applying shellac compared to, for example, pre-catalysed lacquer. For example, where I would normally set the air pressure at the wall to about 50 lbs, I'll reduce this a bit to about 45 lbs. I can also make this adjustment at the gun, but your gun may not have this feature. I find reducing the pressure slightly helps reduce the tendency of shellac to dry with an orange peel appearance.
Of course, if you do completely screw up, shellac is relatively easy to either strip or repair. Slainte.
I can only speak to part of your question. I use an HVLP turbine so they are different breeds. As to the cut etc. I use a 1lb cut only to seal or act as an intermediary between finishes. Nice thing about it though each coat eats into the next and if yu screw it up just use alc over it and take it off. Could be the setup I don't know. The needle combo (1.2) seems right I use that in my HVLP and it works fine again differnet due to low about of air. As to the coats, I put on thin and build up when spraying. It takes time but I've found it works better. One thing you might consider is getting Jeff Jewits book from taunton its extensive and good. another source is Charlse Neil. Go over to his forum and ask him. Both are great at responding. If you call homestead finishes most of the time Jeff answers the phone. (at least my experience) good luck.
Different scenario but when I had bubbles in drying wb finish eventually I discovered it was because air pressure was to high, Can't remember if it was pressure to the pot or to the gun. I haven't had this problem with shellac.
What is the temperature and relative humidity where you are?
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