From WoodworkersZone WoodWiki
Varnishing Your Instrument
By Carl Mills
How could a Stradivarius instrument survive a ship wreck while a Del Gesù might not? Paolo Perterlongo stated in his book that the inside of the Strad instruments were treated with waterglass on the inside and parchment gelatin on the outside, thus water-proofing the instrument. This led to ossification of the plates to near crystallization, increasing the ability of the plate to vibrate.
Experiments that I have conducted with three instruments have not given this same result, but perhaps the passage of time is also a factor. I did not finish the inside of the instruments I’ve made, and that could also be a contributing point.
The first step in finishing your instrument is choosing the type of varnish oil based or spirit based, both have good and bad properties. It does have to be a good quality varnish. Applied correctly and with care, it will give the instrument its final appearance and protect it from damage. The effect on the tone is proportional to the thickness and hardness of the varnish.
If the finish is poor, the instrument will be refused by most players and collectors, even if the sound is excellent. Applying a good transparent varnish is not an easy task. The Builder/Artist in us will determine the final look of the instrument. Remember, your name is going to be it for many years, so the instrument will be used to judge your skill and artistic ability.
My best advice to you is to research the finishing process of wood products as much as possible. Make your decisions based on the facts, but most of all protect your health.
You are responsible for your health and safety. Without them you will end up a train wreck with many expenses that could have been avoided. Read and understand all safety precautions related to the tools and chemicals you use. Obtain MSDS (Material Safety Data Sheet) on all glues and chemical compounds.
Preparation of the instrument
Remove the fittings, fingerboard, string nut, saddle nut, and pegs from the instrument. Protect the fingerboard mounting surface of the neck with tape or card stock. This will help prevent damage and varnish build up on the fingerboard gluing surface.
Inspect the instrument’s surface areas and remove all excess glue. Check the smoothness of the instrument. Scrape rough areas smooth. Check for dents and dings in the wood. If there are dents in the surface, they can be removed or filled.
To remove a dent in the wood you need a small iron that will heat up to 175 degrees F. After the iron is hot, place a small drop of water on the dent and let it soak into the wood. Then apply the iron to the dent with a piece of cotton cloth to protect the wood. Wait 30 seconds and remove the iron. Inspect the dent. The steam created by the iron heating the moisture in the wood will cause it to expand, pushing the dent up. Use a scraper to even out the surface.
Here is a dent in the top plate of a violin with water applied.
The heating iron applied to the top.
The hot iron caused the water to change to steam, the expanding steam has pushed the dent up. I finished with a scraper to even out the ring and pulp wood.
Why use scrapers? Scraping will remove wood from both ring and pulp wood at the same rate, whereas sanding with abrasive paper will lower the pulp wood faster then the ring wood. This is apparent when the varnish is applied and valleys appear in between the ring wood.
Figure 27 is a representation of the sanded and scraped surface of wood viewed from the end grain. The scraped surface is much easier to apply the sealer to.
Clean the instrument. Remove all dust and wood chips. Set up your holding fixture in the vise. This supports the instrument during the drying time. The tapered dowel fits into the end pin hole on the instrument.
Clean your shop to remove all airborne dust.
ENSURE that you set up good ventilation to protect your health.
Use pore filler (potassium silicate [waterglass]) to seal the wood. As the arching is carved, the end grain is exposed in the upper and lower bouts. The pore filler comes premixed in a 2 oz. bottle. One bottle will seal two to three coatings depending on the application method. The pore filler will also accent the grain pattern of the wood.
Use cloth pads to apply the sealer. Wipe from the end block to the neck in the grain direction. Use one cloth pad per wipe; this will give a clean even flow of the sealer onto the instrument. Do not seal under the neck finger board area. Neck stain will be used later to do this. Allow to dry for 24 hours before a second coat is applied. When dry, use a synthetic abrasive pad to give the surface a satin finish.
The shiney streaks on this back plate are high spots. Flatten with a very fine sponge sander, using a light finger pressure.
The second coat is applied using the same procedure. After the drying time has passed, inspect the instrument for thin or missed areas of sealant. Apply a third coat if necessary. Flatten the seal coating using the micro-mesh system wet sanding method (Follow the instructions supplied with the kit). Ensure that no water enters the instrument through the sound-holes.
This is the start of the second coat of sealer. The dust from the sanding will help to fill the open pores of the wood.
Check for high spots, they will apear as shadows or bumps as you rotate the instrument.
The spirit varnish is applied to the instrument by spraying. The drying time is 48 hours in between each coat. This allows the varnish to harden enough not to gum up the micro mesh abrasive pads. Apply six color coats and six clear coats, depending on the depth of color you’re looking for. With build-up of more than 0.05 thickness of the finish, tone output will be lowered due to lack of flexibility.
I start with one coat of amber colored varnish as a base color. The second coat is a clear coat to prevent the different colors from mixing.
This is Birdseye Maple, one coat of amber, 1 coat of clear and one coat of red brown have been applied.
After the first coat, use the abrasive synthetic pad for a satin finish. This allows the next coat to adhere to the surface. Second coat is micro-meshed with 800 grit, then use 1000, 1200, 1400, 1600, 1800 and 2000 grit wet sanding with each successive coating, ensuring that no water enters the instrument through the sound-holes. Allow 48 hours drying time between each coat. Polish with 5000-10000 grit micro-mesh and wipe the instrument clean. Use neck stain to finish under the neck. Neck stain is still elastic after drying, so this allows the neck to flex without damaging or causing cracks to appear in the varnish.
The varnish is flattened with the Micro-Mesh pads in small circles.
The flattened and cleaned back. More color coats will be added to darken the color, finish with four to six coats of clear varnish.
Let the instrument hang in the daylight out of direct sunlight. The final color will come out in about three days, because some of the color may fade. After 48 hours of drying time, polish the instrument with pumice and rottenstone to the desired finish. Reinstall the fittings, use a thin viscosity, even coat of hide glue to attach the fingerboard, string nut and saddle nut.
Polishing and Rubbing
This is perhaps the most tedious and tricky process of the making of a stringed instrument. It can be used to apply a complete finish to the instrument and to repair cracks and damage. The method is simple in its explanation and difficult in its execution. Use of wax based oil is not recommended on instruments. Linseed oil will darken with age. I recommend you use a ½-pound cut shellac as the type of varnish.
Use a three inch square section of lint free cloth folded into quarters or a felt pad. Set up the ingredients for easy access. Wear a respirator to protect your health from the dust and solvents used.
Add a small amount of baby oil to the cloth wetting it thoroughly. Squeeze out the excess oil. The oil acts as a lubricant for the varnish. Place into a lint free polishing cloth.
Dip the polishing cloth into the varnish mixed with the solvent 50/50, picking up a small amount of the varnish.
Dip the tip of the varnish cloth into the rottenstone (as desired), picking up the least amount required for the section you’re working on.
Start at the neck under the finger board location, moving in small circles as you work toward the bridge.
As the varnish is applied, the cloth will start to stick. Remove the cloth and reload it.
Continue the process until the entire instrument is completed. Allow to dry for 7-14 days in a well ventilated, warm, humidity-controlled environment. This allows the varnish to harden sufficiently to withstand the pressure from the bridge feet without leaving an indentation in the varnish.
To complete, rub out the finish with a lint-free cloth. The instrument should have a smooth finish and not be too shiny.
Good luck with your project.