Pap, I never used steel wool on ivory or its synthetics. I gave it a high polish, layed out the design then waxed. Starting with the darkest color (usually black) then waxing before moving on to the next darkest color area. Waxing after each color is added. The wax keeps you from wiping off...
Try a coat of Renaisance wax before scrimming. After each color is added to the design, coat that as well so that the ink is not thinned wihile working on the next area. I used to have trouble with the design fading away as I added each area of color until I used the wax. A trick I learned...
I don't know how thick the plate is but I have found that mounting plates to a block of wood or other solid backup material stops any vibration during cutting...making for much smoother and easier cutting.
I just finished a set of knife scales in Argentium silver with sculptured scenes and some minor gold inlay. Was very easy to work...especially easy on tool points.
I saw a post on the ACGG Forum today to the effect that Tom Turpin has died. Tom was a great friend to all of us as he put many of our engraving photos in the Gun Digest every year. I will miss Tom!