why the coating of the Thin Layer Chromatography plate should not be scraped?

Answer 1

The coating of the a thin layer chromotography plate is of a uniform thickness. Any scrape or mark on it effects the rate of which the solvent moves up the plate. This can cause zones there compound spots are moving faster than in others and hence you will not get a true rf values.

Answer 2

the coating of the thin layer chromatography is silica gel. this is your stationary phase meaning it is the reason that your more polar analytes (spotted samples) interacts with your polar analytes making them move slower than the less polar compounds u spot into TLC. Think of it as the sticky goo that makes the more polar compounds move slower. Your mobile phase is the solvent you dip ur TLC plate in. This is what pushes your less polar compounds further up making them move faster. So if you scrape the coating you scrape of the stationary phase of your TLC plate there would be no way for the spots to go further up or it might blotch at places and so you wont get the right rf values because it wouldnt get as far as it should have.