I removed the old cracked tar form the relay board. What is the best stuff to use on the bottom of a relay board to seal the copper strips?

There was a thread on using Plasti-dip.

Epoxy would be good as it's non-conductive.
I use the WEST system stuff sold at most marine stores cause I do some marine fabrications

Probably the stuff you can clear coat table tops with would work as well
Acrylic I think

I used a product from Smooth-on

Casting rubber

It pours thin. Be sure to plug any holes around the contacts or it will pour through

I used Liquid Tape from Home Depot. I think I'd read a thread about this topic from way earlier and that's what was recommended. Only did it a month ago so I can't vouch for its long term effectiveness. It does go on thin.

Liquid tape

For circuit boards, I use humiseal

I used spray-on Plasti-Dip several years ago - still looks like new. I'd shy away from using anything too difficult to remove, just in case.

Thanks for all the info. I just ordered a tube of Dow Corning 737 sealer, says for use on plastic and glass, from McMaster Carr.

Trot over to Radio Shack (or your local electronics store) and look for some "potting compound".

--DD