asianflava wrote:If you put on the second coat before it fully sets up (not runny but still sticky) it will bond chemically.
If it is cured, you could rough it up with sandpaper so that it can bond mechanically. When you sand, it should come off in white powder and not gum up the paper.
Asianflava is absolutely correct. A couple of additional thoughts...
...a good test on whether epoxy will bond chemically is to see if you can dent it with a fingernail. Some epoxies cure slow, some fast. I'm now using MAS(with the slow hardener) and it gives me about 14 hours before I have to recoat. I used to use West System 107 and the open time was much less.
....regardless of the epoxy used, you need to make sure that it hasn't "blushed". That's a waxy film that occurs on the epoxy as it dries, and can be easily washed off with vinagar. Most brand name epoxies advertise that they do not blush. I've found that to be true. If you have a no-name epoxy (which will probably work well), you need to be aware.
...epoxy can really sensitize your skin and breathing. It can be dangerous if you sand it before it is fully cured..1 to 2 weeks. Be sure to use an appropriate respirator if you decide to sand earlier than is recommended.