Where do you want to do the touchups?

If you want to touch up the graphics to make them stick better then yes, clear fingernail polish will work very well. It's what I use and I am very happy with the results. And you only need to coat the edges.

But DO NOT use a cheap $1 bottle from Dollar Store or anything like that. It must be a well-known brand, and has to be clear, I use L'Oreal Top Coat#110

If you want to touch up the firewall or tank area, then NO, fingernail polish won't do. Thin epoxy with a few drops of denatured alcohol and brush it on. If you do not have denatured alcohol, which is best to use, then regular isopropyl alcohol will work (get the highest percentage you can get for best results). If you can't get alcohol then you can heat the epoxy to thin it, but brush it on quickly as it sets very fast.