Many thanks to our shop owner Mr. Jude Reveley for the instructions and letting me practice. The soundboard is refinished with sanding sealer and top coat. Here we use polyurethane for sealer and acrylic urethane for top coat. Both sides of the soundboard are sprayed with sanding sealer, then only bottom gets top coat. The bridge side will be sprayed with final coat after bridge top is milled to ideal height, notched, and bridge pins installed.
Before any coating, the wood surface is sanded with 220 sand paper till smooth. A good light at the right angle is critical. The shop light at the time was not enough. I left some scratch marks on without noticing. Next time, more light will be placed around the board for a better sanding result. The sanding sealer from ICA is made of solid, catalyst, and thinner. Check manual, ratio, pot life of mixture, and time needed between coats from manufacture.
All products, spray gun, and spray containers are placed on work surface. Once you start mixing, it goes fast. Mark how much of each product needed on mixing cup, fill the cup, mix well, pure mixture into spray container through filter. Spray lacquer thinner to clear the interior of the gun.
Screw the gun onto sealer container, adjust air flow and amount of liquid coming out. Spray two coats on each side of soundboard, 30 minutes between each coat. One hour before flipping the board over to spray the other side. The ideal distance between gun and wood surface is 6 to 8 inches. Make sure the gun is held steadily and arm moving speed is even. This attempt looks better than the last practice a year ago. I didn't even want to sand if off when the sealer dried. Hope to keep it up and improve. Note the blue tape around the rib side. That's where glue will be applied to glue the soundboard in piano. So no finish on the perimeter.
On the next day when the sealer is dried completely, it can be sanded with 320 sand paper and evened out with fine 3M Scotch-Brite. Check with spotlight around the board to see if all the shines are sanded off and surface even. Vacuum clean before moving board to spray booth. The vacuum tip has to be only for finishing job without any contamination.
Next is to spray 2 coats of semi clear acrylic urethane top coat. Again, read carefully of the ratio, pot life of mixture, and time needed between coats. This time, one hour is needed between two coats. It says the pot life is 24 hours. So right after the first coat, the products can be mixed for the next coat, to be efficient.
For efficient and accurate work flow, all products are placed on counter. The acrylic solid needs a good stir before pouring into mixing cup. The mixing cup is marked with how much each product is needed. The stirrer stick must be without any contamination.
Spray lacquer thinner before the acrylic to clear the gun. Pour mixture into spray container through filter, fasten lid, screw on gun tightly.
6 to 8 inches is needed between gun nozzle and wood. Move arm with a even speed across the board. Each pass or strip must cover 1/2 of the previous one. Do not lean to the surface. I once was trying to reach the other side of board while making a big dent with belly part of apron. Bad bad.
After all the spraying, spray with lacquer thinner again to clear gun. Disassemble all removable parts, soak in lacquer thinner, wipe thoroughly. Poke the air and liquid holes with a wire to make sure nothing is blocked. Assemble gun and wipe off any residue. Store or hang at a dry place, together will all the guns or all her friends and family members.