Thanks for clarifying. In that case there are a few things you can try. A glass file will get you the original plastic shine and is really good for flat surfaces. This is the buffing compound I use, which does a good job and works for paints and is safe for plastic. You can also try nail buffing blocks.

The original plastic surface is really shiny because it's so smooth. So if you want it to look like plastic again you have to polish it after sanding with 2500 grit.

A glass file is definitely the way to go to help maintain a smooth edge so they won't break or peel off in layers. There are a bunch of well rated ones on Amazon (like this) for under $10, and there are also ones that will buff your nail so smooth they shine and essentially "seal" the edges (like this). Incorporating those into my nail care routine dramatically changed my ability to grow them longer. I also swear by OPI Nail Envy (here it is on Amazon, but you can sometimes get it cheaper at CVS using their coupons, if you have a CVS near you) as a strengthener. On the hair side of things, I don't have much to contribute. I can grown my hair long even though I am Hypo, but the thinning around my temple area has really been hard to deal with. :(