Learning new things all the time.

So I am finally getting around to working on Disk Brakes
and am working with a TRP Spyke, which apparently being a high end brake, doesn't use spherical washers in the mounting.
Hence I have now learned the importance of having your IS mounts be square and faced, so that the caliper can be nice and parallel to the disk.

I'm a growing bike mechanic, but the place I work does cheap hybrids mainly and we don't have disk mount facing tools.
Any advice on how to handle the facing?
Anyone have some DIY tricks on how to do it, or am I going to have to seek outside professional help (ie, a higher ends LBS than my own)?

noted: personal bike, not a customer's. we inform customers who ask, that we aren't equipped to work on disk brakes.

AFAIK the only brand that routinely uses spherical washers on disc brake calipers are Avid. Think they call it "Tri-align". And that's on Post-Mount, not IS. Or, if you're using an adapter, on the part of the adapter that has the Post-Mount interface.
Never seen an IS mount use spherical washers. Can't see how that'd be possible w/o upsetting the specification way beyond tolerances.
The only disc brake facing tool I've used was for IS mounts. If I had to improvise one of those I'd buy a special machinist drill bit used for drilling countersunk holes for screws with cylindrical heads. They come with a non-cutting center tap to keep them centered. Then I'd mount that in a handheld drill, aim carefully and go at it.
If I was feeling scared or cheap, I'd cut myself some semi-circle shims out of a soda can, insert under the "low" side and experiment with those.

Now that road bikes seem to be moving inexorably toward disc brakes, A facing tool will need to be standard equipment in every shop.
I have an older Var facing tool which is for ISO mounts only, but new versions of Park's facing tool will do post and the new Shimano standard as well.
I have successfully squared up post mounts by mounting my tool in the dropouts and visually aligning the post surfaces with the edge of the milling tool, and squaring up the mount with a file.
So far, I don't do enough disc brake work to justify investing in an up to date tool.

FWIW Post mount from the caliper perspective is universal .

Your ISO adapter has a post mount on the edge..

Avid assumes the washer's thickness (they're parts common to installing V Brake pads)

so perhaps you will bump up a disc size diameter . feel adventurous?

Properly positioning the mounts so they're square and aligned is the framebuilders responsibility. It's possible to correct them if necessary, but a slower, more costly option vs. getting it right in the first place.

Tools exist but aren't necessary if the mechanic has a file and good hand skills.

IMO - this is one area where shops buying tools may be counter productive. If shops get used to dealing with poorly positioned brake mounts, it lessens the penalty for sloppy work. That narrows the gap between better, more conscientious producers and slackers, and reduces the incentive to work to tighter standards.

The better answer to the problem lies in complaining and either returning imperfect frames, or demanding compensation for correcting other people's sloppy work. If sales reports expenses related to the poor work, management will pressure production to solve the issue.

I used a sharpening stone to smooth the mounting surfaces on a friend's trike that uses disc brakes.

Many new bikes just have paint glops on top of the faced mounts that can really screw up alignment. We do just light facing with a file on some bikes to get down to flat metal is often all that is needed for proper caliper alignment... It is a very common problem though.

As long as people are buying frames online from unknown sources, frame prep, which now includes disc mount facing will be a marketable service.

Of course. And those people willing to buy unknown products from unknown sources have to take what they get, and pay others to fix it (or DIY).

My point was that when dealers also become willing to accept inferior product, a critical feedback loop is lost and the overall quality of offerings declines.

A critical element of a healthy market is consumers willing and able to say "that's not good enough". Take that away and you remove a key incentive to strive for quality.