One more thing you might try that I learned after a couple of new drive washers and spinner backplates...

The emory paper trick didn't work at all for me, some great smoke from the drive washer/backplate area when I started the 110 and another ruined backplate and drive washer but that was it. What I do is start the engine, run it for a couple of minutes without getting too much RPM, then shut it down and tighten the prop nuts again. I do this three times with new stuff gradually increasing the RPM each run to full throttle on the last one, and then check after each of the first three flights. I'll generally get a little more tightening each time I do check them and I keep doing it until the thing is no longer slightly loose when I check it. Then I just check it every so often once it's tight.

It's just kind of a problem when you've got an aluminum drive washer riding on an aluminum spinner backplate and the pre-cut grooves in the backplate and the drive washer don't match each other, swinging a 16x6 APC prop in the mid 9,000 RPM range, but the power output of these little engines is just amazing. As you do these run-ins the grooves will actually get a little filled with aluminum on the drive washer and a slight wear pattern in the spinner backplate, this is what cause the looseness. Do whatever you can to avoid a backfire, but if it does happen, make sure you check the prop bolts immediately. It's a bit of a hassle and the drive washer won't look new anymore, but I've not had prop slipping problems since I started using this method.

Just my two cents worth.