You need to mix the 2 parts completely together. Use a work surface that will not absorb any of the mix. If the mix is absorbed by the work surface it may absorb more of one part than the other and will ruin your ratio of resin to hardener. I cut up the plastic containers that a lot of my groceries come in, into small shapes ready to use for each project. Don't mix your epoxy on paper or cardboard as this can absorb the mix. After epoxy glue dries it still needs time to cure until it is fully hard. This can take 24hrs. I leave the mixing stick on the leftover glue so I can see if the glue hardens properly. If the mixer sticks to the plastic surface I can be pretty sure I have mixed the glue correctly.

Alcohol and acetone can remove uncured glue but they can also remove dye so they need to be used carefully. Epoxy does not stick to packing tape so you can use this to mask areas.