Here is the scoop on self-leveling epoxy
I have to say up front that I have no relationship with Precision Epoxy. I'm sharing this information I found out so you can easily use them.
I got another call back from Mike Ramy at Precision Epoxy to answer my questions. They have updated the web site to make it clearer about this product.
Their product: SC-15P "Steel Casting Epoxy" is specifically designed for self-leveled beds for linear rails. It has the viscosity, steel-bonding, and toughness we want.
All you have to do is tell them the area to be covered, and they will make up a kit to do the job. That includes a part A can with enough room to hold the part B. You pour in part B, mix with your drill and the supplied mixing blade, and pour. They recommend a 1/4" deep pour.
He said not to skimp on the width of the pour. My beams are 3" wide, and the linear rails rails are about 3/4" wide. He told me not to make the effort to have a smaller width that 3". He doesn't know what the minimum width would be, but more width gives better flow and leveling. He also recommends three temporary bridges 3" wide between my two 5' X beams. This allows cross flow so that the two beams to be leveled with each other. The bridges at the ends would be 6" in from the ends of the X beams. You end up with a glass-like flat surface. No sanding required!
Lead time: They recommend a 2-week lead time. They get busy at times, so it may take a while to get your order in, but they like to call you back and talk to you about exactly what you need (Mike prefers the phone to email).
Support: Mike gives you his cell phone number. You can call him 24/7. That seems pretty impressive to me! I also didn't get any "that's too small a job" attitude. I told him it would be a couple of months before I order, and he was fine with that. :cheers:
To sum it up, this seems like a no-brainer to me. Exactly the right product, and excellent support.
P.S.
Mike told me about another method you could use if the bottom rail surface isn't perfectly flat (I assume my profile rails will be). You bolt down the rails with a gap using shims/washers perfectly flat using some reference. You have the rail itself coated with Vaseline. You pour epoxy to fill in underneath. You can take the rail up again if you need to. I told him that wouldn't work for me for two reasons: 1. I don't have a reference, 2. The profile rails are not straight. Anyway, I thought I would pass along that method.