[Arran]

 I have a Canadian 6a7m Rodgers tube that has spray shield on it, now I can replace it with a 6a8g but have to add shield which I can do but is there a product that I can apply on the tube instead like original? These tubes a hard to find. The tube I really badly need is a 6f7m , if I cant find of those I will have to rewire tube socket for a 6b8 tube

Fred;
 A 6A7M can be replaced with just a 6A8 metal type, no G. Another option is to add a Goat shield to a 6A8G. There are two substances that may work on a G type octal tube to recreate a spray shield, one is slip plate which our own Bob Andersen uses to replace the aquadag on picture tubes, the other is a type of spray paint that some auto parts stores sell to repair rear window defoggers. The problem with the slip plate is that it is not that durable, I would guess that the rear window defogger stuff may be more tough, apparently British and European radio restorers use the rear window defogger paint to repair Phillips/Valvo/Mullard and other tubes with spray shields.
  As for the 6f7M from what I can find a 6F7G and a 6P7G can be made to work since they are all triode-pentodes, though you may still have to rewire the socket to use a 6P7G, or make an adapter in the case of the 6F7 with an older seven pin socket and an octal tube base. Perhaps you were thinking of a 6B7M which is a dual diode-pentode, that can be replaced with a 6B8 or 6B8G by rewiring the socket. By the way, the name of the company was "Rogers Tubes LTD." no "D" in Rogers, just as "Rogers-Majestic LTD".
  I have a Canadian Marconi Tube manual from 1946 that has specs listed for most of the Rogers type tubes like 2X3s, 2Y3s, 6H7S, 6H7M, 6X6, etc., so I will look up some of these types and see what the cross reference is. I also have a few copies of a Rogers tube substitution pamphlet around here but I don't know where I put them.
Regards
Arran