RMS 36 is equal to a file finish or hone, it's not the finish that is all that important its the flatness. Check it for high spots near the edges of the block, many shops offset their milling heads half a degree or so to prevent the tool cutting on the backside of the cut and spoiling the finish. I always take a fine cut file and scraper to the head surfaces to make sure they aren't slightly high near the gasket surface edges. You would be surprised to find many are, even if it's only 0.0002" or so. Chamferring the bolts holes is a detail that should not be missed either.
Most people think a file and scaper cannot achieve as flat a surface as a milling machine but in actuality it's the opposite, using the proper draw filing technique and with some experience you can make a surface a lot flatter with simple hand tools than a machine can. For example lathe ways are all hand finished on the real expensive tool room models.
Check it out, these details are what separate the men from the boys when it comes to engine building. I'm willing to bet your heads are high on the edges of the casting which is causing problems because now the head is a spring you need to compress with bolt torque to get a seal, thats why it blew late. Took a few hot and cold cycles to stress the head bolts out and unspring the gasket surfaces. Just a theory.
The proper technique with head surfaces is to draw the file down the length of the gasket face and then at 30 degrees from both sides to it to get a nice crosshatch pattern all the way across. What you are trying to achieve is removing any highspots without removing any metal from the low spots.
You'll see what I'm talking about when you try it.
