John Maye's notes on finishing & bending

  
John is a friend of LINT whose shop is in Norman, Oklahoma. He thoughtfully provided us with his thoughts on lacquer finishing and side bending.
Click here to visit his web site.
First off this is by no means the only ways to approach finishing a guitar with nitrocellulose lacquer. This is just the method that I have come up with from research, trial, and a lot or error.
The first step in finishing the guitar would be wood preparation. I sand all my guitars up to 220 grit sandpaper. You can actually stop at 180 grit or even take it up to 320 or higher but I find that 220 is sufficient and seems to work well for me. You will want to sand the surfaces to be Finnish until there are no visible scratches. One way to tell if there are scratches is to slightly wet the surface by taking a damp rag and wiping down the surface. Any heavy scratches will show up well under water and light.
The first step in the spraying process is to lay down a very thin layer of sealer. There are many different types and brands of sealer you can use but for McFadden’s Lacquer the best sealer I have found is either Parks sealer or Minwax sanding sealer. Both of these brands have a high solid content so they build nicely, and fast. I have used Parks quite a bit and have gotten good results. I spray one coat of sealer very thin over the entire guitar neck and body (with the exception of the fingerboard which has been taped off). The reason why the sealer is necessary is because although lacquer adheres to itself and to sealer well it does not bond to raw wood that great. Sealer on the other hand does stick to wood well.
After the first coat of sealer it is time to pore fill. This stage can and is skipped by many luthiers and I think it is more or a personal decision. I have done it both ways but prefer to fill the pores with a paste pore filler because I feel it helps cut down on shrink back of the lacquer after the finish is complete. I have been using LMI micro bead pore filler. You will want to pick the appropriate color of filler. For the mahogany guitar shown I am using the dark color. You want the pores to be just darker than the wood. Even though this is designed for water based finish if you let it sit long enough then it interacts fine with the oil based stuff.
So to apply the pore filler you simply apply the paste directly on the guitar and then spread it across the guitar in small segments using a rag or spatula. You will want to scrape it across the pores, which pushes the filler into the pores. Cover the entire body and neck (except for the soundboard as there is no pore filling necessary, except with a hardwood top such as mahogany or Koa) and scrape it flush so that there are no large globs of filler left standing. Let it dry for 2 days. The instructions say 24 hours but since we are using a oil based finish instead of water based then I like the extra time to let the filler cure. After the two days sand or scrape the filler level and the excess off and then your ready top move on.

You will then want to spray one more coat of sealer due to the adhesion issues discussed earlier. This coat can be slightly thicker. After the 2nd coat of sealer it is time to move on to the lacquer.

I have been using McFadden’s lacquer for about 5 years now and have been very pleased with the results. It is relatively easy to apply and easy to work with. I spray the lacquer un-thinned so that it builds quickly and minimal layers are needed. The only thing I add to my finish is retarder. I do this because it is rather humid in Oklahoma and without it the finish would blush. I have my gun set to spray a large pattern. The pattern is about 8 inches wide. I spray thin coats of finish about one and one half hours apart.

One big factor while spraying is the envoirment. You will want a dust free area. I have found if there is one black dust particle floating around it will always gravitate to the top of the guitar and never seems to stick where the pickguard would go, unless it is a clear pickguard. You will not get a good finish unless you keep the area you’re working in dust free. It needs to be dust free….should I say it again? I think you get the point.

When spraying the back I overlap the spray pattern halfway what I just sprayed. It takes about 6 passes on the back and front and about the same on the sides. I always spray the back first then the sides and then the top. No real reason except it seems to work well for me.

I spray three coats the first day 1-1/2 hours apart. I then let it set overnight.

The next day I sand with 320 grit sandpaper. I have been using Norton’s sandpaper because it is cheap and does not clog really fast. Although I do go through quite a bit of it. The idea with the sanding at this point is more to rough up the surface to give the next coats something to adhere to more than leveling.

Spray three more coats of finish again 1-1/2 hours apart.

The next day sand with 320 again making sure to get things level. Not smooth as you would cut through the finish and that would be a bad thing! Just making sure that the finish is level. Then let the guitar sit for 2 days. During this time you are letting the guitar gas-out. The lacquer is releasing gasses from the finish. Also during this time you will want to drop fill any pits or gaps in the rosette/binding. I do this by taking a small container and putting an ounce of lacquer and letting it sit in the container for a few hours to help it to get a little thicker. Then take a toothpick and put it in the lacquer and then use it as a drop tool and apply it to the sections that are low or have gaps or pits.

After the 2 days are up scuff sand with 320 paper again and spray three more coats. Let it sit overnight.

The next day I do my fret jobs. Some luthiers do them before and some after finish but I find it best to do them in the middle. This is just my way of doing things. After the fret job take the 320 paper one more time and sand the guitar until 90% of the pores do not show. You are shooting for a dull sheen. If you see a shiny spot that is a pore or low point and try to get it out without breaking through the finish. A pore here and there is fine. Do not risk breaking through the finish to get one pore! It is easier to get the pore later than it is to touch up a break through.

Same day as the fret job I spray three more coats. The next day I wet sand with 600 grit wet/dry 3M imperial sandpaper. I use a Detroit national pneumatic in-line finish sander. These guys are really expensive but well worth it! They cost about $400-$500 each. They save so much time they will pay for themselves very quickly. I sand until there are no shiny spots anywhere. This only takes about 30 minutes with the sander. As lubricant I use water and liquid dishwashing detergent, you can even use dove to keep your hands non-wrinkly. Use plenty of water/soap. You do not want the finish to build up as it will then start making scratches deeper than the sandpaper, and it takes more work to go back and sand them out.

After the 600 grit I spray a finial 3 coats of finish. It may seem like we are spraying a lot of lacquer here but if the coats are thin like they should be the final thickness of the finish will end up around .005-.007 of an inch. Pretty thin!

The next day wet sand with 800 grit sandpaper. Same process as before, no shiny spots.

Then you will want to set the guitar aside to cure. Most of the times this includes just putting it aside and forgetting about it for a month. I have started using a curing box after an idea I got from lance McCollum. (Thanks Lance!) It has saved me tons of time!! The curing box is just a box with black lights in it with tinfoil on the walls to reflect the light. Black lights emit a UV ray that helps cure the finish. I let the guitar sit in the box for about 30 hours. Then I take it out and let it sit for another day or two before touching it again. After this time in the box and out in the open the lacquer seems to have shrunk back 90% of what it will. One caution about the curing box though. Because it does emit a UV ray it will oxidize the wood. That means if you want that really white Englemann spruce top do not put it in the box or else it will end up with a slight toned tint. If you are going for a vintage toner look however this is the best thing you can do! You do not have to tone the top and mess with fake toner when the curing box gets it there naturally.

Ok so after the couple days out of the box I wet sand with 1200, 1500, and 2000 grits. This time you cannot really tell if you have got all the previous scratches out but technically you can go to the buffing wheel after 1000 grit. This way just saves you some time.

On to the wheel. I am using Menerza compounds. I use a medium compound and a fine compound on different wheels. I use a liberal about and reapply it every few minutes. The compound does the work for you. When buffing on the wheel do not linger in one area too long or else you will burn through the finish! Take semi-fast passes overlapping each one halfway across the guitar. Always buff the lower quadrant of the guitar because if the wheel catches the edge of the guitar it is likely to yank the guitar out of your hands and on to the floor, and we know that would be a really bad thing. After buffing which seems to take me about 1-2 hours you can take and use a really fine swirl remover or scratch remover to get the buffing haze off. Keep buffing and removing haze until your done!
Sit back and enjoy your work….Good job! Now on to gluing that bridge.
Side bending: 

I get lots of questions e-mailed to me about bending sides and while I am always happy to answer the questions to save some repletion here are my methods to bending sides that I learned/worked out in Maine working under Dana Bourgeois. Dana did all the hard part I just fine-tuned the temps and side thickness to the size I thought worked best. Ok so here it is.

For side thickness it will vary from model to model and for each species of wood as well. But as a general rule here are the thick nesses you should thin the sides to.
Dreadnaught .085-.090 ---- Slope D .085-.090
OM .080-.085 --------------- SJ .080-.085
anything with a cutaway (unless it is a sharp cutaway) .075-.080

The best bending method I have used so far is to use TWO silicone heating blankets available from LMI/Watlow and other places too. Two blankets ensure that you get quick even heating across the whole piece of wood. One on top and one on bottom with the wood sandwiched between two pieces of spring steel .010 that is essential when bending to avoid cracks and reduce spring back.

There are a couple different things to keep in mind while actually bending the wood. 1st is the point in which you can start the bend. There are a few ways to do this but I approach it this way. The boiling point of water is 212f so at about 225-240 you will start to see steam coming from the blankets. When you see steam you want to bend. Since the water is steaming the steam loosens the fibers in the wood and therefore makes it possible to bend the wood. As a steadfast rule you can start bending at 240f if you see steam or not. If you do not see steam it may be because you did not put extra water on it, and that is ok as long as you put enough on. Enough would be a thin coating over the whole side but not dripping off. You can know what temp you are bending at by buying as digital thermometer from Wal-Mart. The kind used for cooking. They have the long stem that would normally be put into a turkey or something like that. They read temps up past the highest we would use.

So at 240f you have started your bend. You move somewhat slowly..not so slow that by the time your almost done there is no water left in the fibers as that can run the risk of scorching, but slow enough so that you don't break the wood. On highly figured woods such as quilted mahogany use extra care as they can be very tough to bend. The process I use for bending is to bend the waist first and then the lower bout and then the upper bout. Not the only way but the way that suits me best.

I regulate my temperatures via a potentiometer or a dimmer switch. I then can bring the temperature up the mark that I want and then keep it there for as long as I want with less plugging in and unplugging of the cords, but either way works.

So the waist press is down, and the sides are also curved. You will want to bring it up to a temperature and hold it there for around 5-7 minutes to really 'set in' the bend. Each wood has a different temperature to go to and some woods are less prone to scorch than other so here is a list of woods commonly used and what temperature to bring them up to:

Indian Rosewood: 300f Mahogany: 310f
Maple (bigleaf): 290f Maple (European): 280f
Brazilian Rosewood: 305f Cocobolo: 310f
Koa: 300f Paduk: 310f
Walnut: 300f Other Rosewoods: 300f

You will want to keep the wood at that temperature for about 5-7 minutes. Then unplug the wood or turn off your dimmer switch and wait for 30-45 minutes before taking the wood out.

With these methods I have bent hundreds of sides and using them should help you to feel confident in your bending and not afraid that you are going break a side.

Again these are just my findings. Above all be flexible and be prepared to find your own methods. I hope this helps!