I did a search and found a few threads, but I'm having kind of a hard time getting all the tar off the underside of the relay board to a clean surface.

I was able to chip a lot out. Then I tried a heat gun which really just turned it into goo -- didn't make it easier to remove. Then i soaked it in mineral spirits, which kind of gummed it up...I was hoping it would dissolve it.

How can I get the underside pristine? Other chemicals (isopropyl alcohol)? Some kind of media blast?

dry ice, gloves, chisel

I'm hoping there's a chemical alternative to avoid mechanical damage to the board and traces.

Submerge the whole board in Acetone for a week, then scrape off what you can, then soak again for another week. Scrape. Repeat

ok

It werks.

I stick my boards in the freezer overnight before I start scraping. It makes it a lot easier to remove.

Try Xylene, we use it at work to clean our tar based flashing and mastic. Just to be clear, I have not tried this on a relay board, so I am not sure how well it will work, but it is worth a try. I've also found that diesel fuel really eats tar. Most options are going to be messy though. Tar is nasty stuff. Hope this helps.

I just did this the other day.

Heat gun to soften the tar, then a wide bladed putty knife to scrape it off. Do an inch wide segment at a time, and once you get the bulk off, then use a fine putty knife and heat to clean up the rest.

Rich, if you can melt the tar with the heat gun, you already resealed it.
I know you're trying to get it of and I assume you want to recover the board. But if you can just reseal it then I think you're done

I'm trying to remove it so I can solder the rivets and fix a couple of non conducting traces.

Goof off seemed to take it off. I heated mine up and got the big stuff out, then rinsed and soaked in goof off. Still a mess, no matter what.

Hugh
I didn't know those boards can get so bad. I just thrown away a spare board I had that had all the tar fallen off but the copper traces were fine. The traces are quite thick so brakes in them means they are really gone.
I think I have another nice board somewhere. I'm not sure if there are differences between early/late/djet/ljet and not sure which one I have anyway. PM me if you need it and I'll try to dig it up.

Berrymans B12 really loosened the tar up, still a mess but the tar is almost a liquid.
I bought a board just to experement seem to work for me Wes