This is your starting point. Rust is minimal, and there are no rot pockets. The paint is flaking, the sheave may squeak, but it turns. Your job is to get to all these incipent problems and keep them from getting worse. Select your block with care. If you absolutely need or want to do a block with worse problems, find someone to instruct you.
First, take off the keepers. Determine which side of the block the pin goes in on. This is usually fairly easy, the pin is rounded and often does not go all the way through on the side that goes in. The pin has "ears" and/or is thicker on the side it comes out on.
Sand the top of the block cheek on the side the pin goes in on. See if there's already a number and letter code stamped there. If so, use it. If not, mark the block there. There are letter stamps on the bench. Choose one number, one letter.
Our blocks are all individuals, they are not mass-produced. The parts from one will not fit another, nor will the parts from one side fit the other. The block must be put together exactly as it came apart and this is where careful attention will save you, and the staff, headaches later, especially if you are doing multiple blocks. Mark the block strap and the sheave with a sharpie marker, on the same side as you mark the shell.
Take a plastic cup and label it with the block number. Put the keepers in the cup, and the screws. If there are any other loose parts, put them in the cup too.
Put the block, with the side the pin comes out down, on the board with a hole in the center that you'll find alongside the bench. With a drift and hammer, knock out the pin.
If the pin won't come out, there are several things to try.
On the stringer next to the block repair area, you'll see a line and a come-a-long set up. Tie the block to the line, making sure to surround the entire block by passing the line through the strap. If you pass the line through the shell, you risk pulling the shell apart. Attach the come-a-long to the hardware on the strap, or the strap itself.
Crank on the come-a-long and it will pull the strap out of the shell.
Mark the sheave as above.
Immediately (and I mean this) put the strap, sheave and pin together exactly as it came out of the block. Tape the pin into the strap so it can't come out and tag the shell with a tyvek tag. Keep the shell tagged for the entire operation, including painting. Lay out each block as in the example above, especially when taking apart multiple blocks. This will save the crew the hassle of sorting out block parts later, and will speed up your work immeasurably.
Inspect the sheave, strap and pin for rust and damage. If you find any, and it is slight, clean it up with a wire brush and apply rust converter. If you find extensive damage, either get help or put the block back together and choose another block.
In the tar room, in the workshop, there's a blue locker labeled "Acids." Get the rust converter, either Gempler's or Qurox. Pour a small amount into a cup. Wear safety glasses and gloves while using this stuff, it will turn your hands black. It also isn't good for your eyes. If you wear glasses, unlike paint, rust converter will not chip off plastic lenses. So do yourself a favor, wear safety glasses. There are acid brushes in the paint locker down in the hold, or in the scaling tools locker in the workshop. It will turn black where the rust was as it dries. NEVER, EVER POUR USED RUST CONVERTER BACK INTO THE JUG! You will contaminate the whole jug. If you have leftovers, find a rusty spot on the hull and smear it there.
Read the directions for more info, but in any case, rust converter has to sit a minimum of 24-48 hours before being coated with anything. So do the next steps that involve rust converter and set the block aside for that long.
Take one of the tiny wire brushes and chuck it up in the drill press. Blocks are pinned together at the sides, and the cheeks, and the pins rust. You'll see the rust, as you do in our example block. Sometimes there are wooden plugs on top of the pins, sometimes there's just putty, or nothing. You only have to do this step if you see rust. Pop out the plugs, if any, and use the wire brush to clean out all the rust and putty you can.
Paint all the rusty pins with rust converter.
Sand the block shell. Get all loose paint off and fair out what's left. Don't remove good paint, just rough it up. Remember, these blocks could be up in the rig for years without service. Prep them as well as you can, don't give the water any way to get in there and destroy all your hard work. Prep inside and out, with a sander, and by hand.
In the paint locker you'll find filler. Wear gloves and work quickly, it sets up very fast. Mix it according to the directions, making sure not to get the hardener in the filler in the can, or you'll ruin the whole can. Fill all holes and cracks in the block, including the screw holes and the cracks between the cheeks and swallows. It may take two applications to get all sides of the block, but this stuff dries in 20 minutes, so you can easily do this in a shift. It runs, so use black tape to hold it where it may run. Overfill all the holes, the putty will shrink a bit as it dries, and refill a second time if necessary.
Sand the filler down when it is completely set. Run the large fan next to the block area and use a respirator, do not breathe this stuff! Refill if necessary, remember that any holes or cracks you leave in the block are places where water can get in and rot the block shell.
Hang the block on the bars next to the workbench. Prime it inside and out with water based primer (in paint locker) if you've exposed any bare wood in the sanding process.
Topcoat with Devoe QD white (in paint locker). One coat inside the shell, if you had to sand there, and at least three coats on the outside of the shell. Go over the shell and catch all your drips, sand between coats. The smoother the finish the longer it will last. Read the labels on the paint to get the time between coats.
When the last coat of paint is dry, you're ready to reassemble the block. You already have a line of block hardware and plastic cups laid out, right?
Take apart the strap, pin and sheave, making sure you keep them oriented properly. If the pin is rusty or scaled, or was at all difficult to remove, clean it on the wire brush on the grinder up in the workshop.
In the locker is a spray can of Eureka fluid. Spray the strap on the outsides, where it meets the block shell.
Put the strap in the slots at the top of the block and use a mallet to tap it into place. Look through the holes at the center of the cheeks and when the strap holes are matched up with them, stop.
Put the sheave in, line up the holes. Coat the pin and the holes in the sheave with Nevr-Seez, also in the paint locker. Be generous with this. The next person to disassemble this block will thank you for it.
Making sure that you have the pin going in the proper direction, use a mallet to tap the pin back in. Make sure it isn't sticking out on either side.
Use a pricker to make pilot holes for the keepers and put them back on. If the screws are at all worn, replace them. Never put anything but bronze screws in a block, to avoid the possibility of rust or electrolysis.
Label the block and rack it.