Doug

Freezing:
The boards have probably gone through all the weather seasons. The previous owner might not have been bothered by their condition, and that is why they left them to remain in that state. When tar stains are a huge bother to you, you can consider using ice to freeze them so that the tar can harden.
Alternatively, if you do not have that much ice and are not in a hurry to use the materials, consider waiting for the winter season to make the freezing easier.
Tar freezes when exposed to frigid temperatures. Once hard, use a scraping tool to remove it gently. It does not come off all at once, and you have to keep repeating the process.
When the residual tar is out, apply an oil-based solvent and let it rest for a while. Water-based solvent is unsuitable as it will soak on your boards, making it impossible to clean. Use a cloth to rub the solvent to remove the tar stains. Your board will definitely look different, even if not as clean as new.

A Hot Air Gun
It is another brilliant way of removing tar from reclaimed boards. However, when using this method, you should be careful not to start a fire or leave scorch marks behind. Use a hooked paint scraper to have better results.
Because fumes will be produced while using this method, ensure you work from an open area to avoid consuming them. Wearing a mask is another precaution you should take to ensure you do not breathe the fumes. Working outdoors also helps in preventing putting your property on fire.

Slicing:
If your reclaimed boards are thick, consider slicing ‘’1/8’’ using a band saw. It helps get rid of the tar and remains with cleaner boards. You can then sand them to make them smooth and proceed with your roofing project.
But, if your boards are not thick enough, the method may not work for you since the remaining material after slicing can be too thin to use for roofing.

Sanding:
Since the board is already dry, you can consider sanding to remove the tar. Start with a coarse grit to work on the excess tar residue as you progressively change to a finer grit. 40-grit sandpaper is good to start with.
Finer grit, 80-120, helps with smoothing the board and getting rid of any scratches that may have formed.

Painting:
Most Saint Louis Roofers will say you have to forego the natural aesthetic created by the roofing boards when the stains are too stubborn to remove. After all, the material is reclaimed, and you tried your best to make it look good.
Sand the boards to make them smooth and even. Then, choose a paint that will complement the part you want to use the boards on and paint. Allow to dry and use as intended.

Disposing:
Some reclaimed boards may have tar from the 1900s that contained asbestos. The substance poses a health hazard when you get exposed to it. Instead of working on such boards, consider disposing them off appropriately and getting new ones.
In other instances, the reclaimed boards can have too much tar that is not worth putting in the effort. Trying to improve them becomes more expensive and time-consuming.
You may not necessarily get quality roofing boards with a small budget, but they are better than risking your health.