Hi there,
You have a really cool and unique project idea!! It sounds like you're going to be running an awesome experiment!
The other experts have provided some great advice regarding how you can go about your procedure, and I suggest looking into the links they provided, as they have some great information that you might find to be very helpful.
As far as the procedure goes, because the other experts have given you a lot of ideas, I just wanted to pitch in with my opinion so that you can decide yourself on which method you want to go with. These are just my thoughts!!
https://www.wikihow.com/Paint-Press%E2% ... ingernails
Here's a really great step by step guide that explains how you can paint fake-fingernails for your project in an easy, hassle free way.
As far as durability goes, try to think about what you are trying to model in your project. From what I gathered, you're trying to use this experiment as a model of how durable nail polish is in real life, when someone wears it. So, according to this, I would not soak the nail polish in any kind of water or remover before you do your scratch test, just to keep things as close to "real" as possible.
If it's okay, I've added a sample procedure that you might find helpful. Feel free to use as little or much of it as you like, make it your own, or just select certain steps that you like!!
1. Prepare fake nails for painting and label the backs with the letters corresponding to the paint-type they get (include more info, this is where you do the wiki-how steps!)
2. Paint 3 fake nails with nail polish A (expensive)
3. Paint 3 other fake nails with nail polish B (moderate)
4. Paint 4 other fake nails with nail polish C (cheap)
5. Allow the nails to dry for 24h
6. Using a piece of sandpaper, count the number of swipes that it takes for each nail to show "scratches or damage" and record this number in your data table
7. Be sure to take before and after pictures!!
This is just an idea, but it seems like it could be the simplest and reality-modeling experiment that could work for your testable question.
Let me know what you think, and if you have ANY other questions, please let us know and we'll be so happy to assist!!
Happy experimenting and stay nerdy!!
lmp1341