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The grain should have been filled with a pastewood grain filler prior to using shellac. Shellac has a natural wax in it and can cause adhesion issues with finishing products. From where you are I would sand the shellac and put a coat of Zinsser Sealcoat on it. It's shellac but has been refined more to remove the wax. Then you will need either a clear or a pigmented grain filler. There is Aquacoat which is clear and Mohawk makes a mahogany grain filler. Most grain fillers come in a natural color which is nearly white. It would make little white lines all over the wood. Another option would be to purchase a grain filler at a real paint store such as Sherwin Williams. It comes in the natural color but they can tint it like they do paint. Once the filler is dry lightly sand the surface to remove any grain filler which may be on the surface. Another note: Once you start back with the shellac allow it to dry overnight before doing any sanding. The solvents in the shellac will make the grain filler swell up and if you sand it immediately when the grain filler shrinks the wood will look grainy again instead of having a level finish.Today we sanded the case, had brush buildup and given it is Mahogany, it has a lot of open grain that we want to fill in.
Some areas are back to the wood with the grain filled, looked odd.
Saved the surfaces to smooth.
Shot it again, this air brush has a good pattern, it has a small bowl so easy to use, gave it a couple coats and it came out real good.
It has some spots where it still needs a little work, steel wool should be all that it needs to prep for next coat, will let it gure until next weekend so it should be good to work.
It worked very well on the columns, air brush is a must for larger areas or odd shapes.
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Bubbles in a finish normally indicate the finish wasn't thinned enough.Have been working on the finish in small time slots.
We needed to get the brush marks and grain peaks leveled down.
The shellac seems to sometimes react to a dust spec or other unknown item resulting in a bubble.
The airbrush allows TINY amounts to be added to build up a thicker coat and once it hardens it is glass like, except for the bubble.
We have a shaving tool that was a lathe bit, 1.5 wide tool steel with a chisel edge that can carefully shave just the bubble.
Then with a pencil point we can carefully add shellac to the bubble for it to fill in the area.
First photo is after first pass, next is ready for touch up, then touched up followed by our tool.
a good sharp chisel would do same.
Reduces sanding time.
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Shellac is a natural material secreted by an insect, the lac bug. You can over thin shellac but it won't hurt it, it will just cause you to have to use more coats. It is necessary though to thin the shellac to fit the capacity of the sprayer being use. Being this topic is using an air brush that isn't much volume so it would be necessary to over thin. It's just very important to have the shellac apply wet. Otherwise you get dried overspray in the surface and lap marks where you start and stop. I would only use an airbrush for the smallest touch-up. It is possible to choke a full size sprayer down to where it's almost like an airbrush. This is what I normally use and rarely use an airbrush.Been tinkering with this and making good progress.
This is a long one, almost finished with the finish.
As mentioned before, we are NOT a painter.
But we have sprayed PAINT many times with a detail gun to larger things.
Some folks have made observations regarding orange peel look and possible water in the line.
The airbrush KIT has a matching compressor with a dryer so dry air.
The question back to those comments is a simple one, assuming they are skilled painters, their observations may be right on...for PAINT, do they have experience with shellac or are their comments general based on paint and not shellac.
SHELLAC IS NOT PAINT!
It is an odd material to use.
With other finish products the applied material cures and you can add additional layers and these are stand alone layers that do not effect the previously applied layers.
Most also take some time to cure.
Shellac is magical, the previous application may be fully cured, but given Shellac is flakes mixed in a solvent, the application of new material causes the existing material to UN-CURE.
If you mix it too thin you are applying more solvent than shellac, the existing material softens up and the new solvent evaporates and the actual amount of applied shellac is minimal.
There does seem to be a magical viscosity where you can get enough material on the surface to "flow" or level to reduce the orange peel effect.
it starts to cure as soon as it leaves the brush, the thicker viscosity will build up faster but it does not seem to level well.
You are limited to the viscosity that your device can handle.
Our airbrush has a top mounted bowl so the gravity feed will allow for thicker material.
The trick is to get it thick enough to allow some buildup of material and thin enough so that a large area of the surface can be wet and thin so it will not be bumpy.
Thick or thin?
Fan or spot?
If you apply a thicker viscosity you can build up the finish more.
However, it takes DAYS to cure well.
we found out the hard way, 3 days after we sprayed one side of the lid we flipped it over to do back side, it felt hard but the weight of the lid caused it to settle into the surface it was resting on.
Even at a week it can feel tacky.
Our brush has 2 spot nozzles and a fan unit like a regular gun.
The spot nozzles work well for thick or thin.
The fan needs a fairly thin viscosity to properly atomize, good for paint, not so much for shellac.
The fan looked like it was going to produce a great finish...but it looked like it was spraying cotton candy.
Paint cures or dries, shellac dries as the solvent evaporates out.
Maybe on a cold day it would work, but in our upstairs shop at maybe 80 degrees the solvent would evaporate out as applied.
Hold close then it sticks, but at proper distance cotton candy.
Our spot nozzles atomize much less so the material sticks.
It still matters what distance.
Close up is good, back off not so much.
We also learned to spray towards new area.
The gun is not 90 degrees to the work so it is at a slight angle.
This causes the air flow to go away from the brush at that general direction.
Even with perfect distance you still get some cotton candy, so you start close to you, work towards the opposite side so the just applied material does not get and overspray.
We found that you work small areas of the piece, start at edge close to you, go sideways first, then front to back.
Do another area next to this, same thing, work FAST!
The goal is to get a thin coat over the whole surface.
Once done START OVER at the edge closest to you, go sideways only, now go slow with about 50% overlap working to opposite side.
This keeps the cotton candy away from the finished area.
Do NOT worry about the cotton candy, any that was there from earlier passes will be pushed into the finish by the last pass.
When finished it may have so.e cotton candy on it, do NOT mess with it, let it cure for a WEEK.
a soft brush takes it off.
In flake form the shellac shouldn't be going bad. What are you using to filter the shellac once mixed?We are using flake, and yes, we can spray over the stuff and it lays back into the finish.
It takes some experimenting to get results.
Advantage with flakes is if you thin too much It is easy to thicken it.
It seems to cure fast, but it is not hard, it could be the surface skins over blocking the path of evaporation.
Shellac never mixes completely and has impurities in it as well. It should be filtered through a cotton cloth before using to rid it of these unwanted materials. It's possible the problems you are having is some of these materials. Shellac is a dirty business, the bugs deposit this raw shellac all over tree branches and such and people break it off and break it up into small pieces where they just rinse it off and ship it. This raw shellac contains dirt and bits of the tree branches. There is just no telling what contaminates might be contained in raw shellac is why it should be filtered.Filtering?
Not doing that.
We are applying multi-coats at one time, trying to have the whole surface WET at same time so it cures with good surface.
The last coat came out fantastic, but we got too much of a hurry and brushed off the cotton candy, left some tiny scratches that are almost invisible.
It has been fun learning, the suggestions here help with things to try.
Next clock may be a Sonora, we will see how that goes some time later.
We still need to finish the back and lid.
The columns have shrunk a tiny bit, we are thinking of using a spacer made from file folder or other material, then filling the gap with shellac, after we are certain it is finished.
Photos will come...