OK, here's my follow up. I didn't get around to doing anything more with it last year, but better late than never!
I put EZ Slide (TSC doesn't sell Slip Plate any more, but have a product called EZ Slide that comes in green and white can that looks a lot like the Slip Plate can, so I think it's the same stuff with a different name) on the LandPride 6' mower a couple weeks ago. I tipped the mower up so the deck was vertical, pressure washed it, wire brushed it (using a wire brush on an angle grinder), then brushed on 3 coats of Slip Plate/EZ Slide. I let it sit for a week to dry. The Slip Plate/EZ Slide started coming off within 2 hours of use and the grass buildup was less than before, but not a big difference. Now I spray it with WD40 (don't use the small cans, get a gallon and one of their pump sprayers and it will be much cheaper) and it works better than the Slip Plate/EZ Slide and is much cheaper and faster and a lot less work.