Hello, first it depends what type of resin your painting. Is it polyurethane or polyester? I normally use polyurethane. I do a lot of high detail casting and painting. First I make sure the resin cast is completely cured or set. Make sure the resin set up correctly, this is important because if the resin is not cured it can continue to offgas and weep out moisture, oils.etc. this will make painting it a nightmare.
Next I would clean the resin with a diluted mix of acetone and water. Be careful with the tone because it can melt your plastic and soften details. If you dilute it with water it works great. Clean your casting quickly, then make sure its dry. The acetone really removes all the left over silicone oils and release..etc. Next I would use warm soapy water. Let dry completely.
After the piece is fully dry I prep the surface. Now depending on your object you want painted you might need to lightly scuff the surface. This is to help you paint adhere correctly. I use a very soft scotchbrite pad " white" color. Once surface is scuffed blow off any dust .
Paint. I like to use acrylics when painting resins. Acrylic is plastic paint so its perfect for painting on plastic resins. You can use oil paint, automotive paint. If you want. So once I'm done painting I allow the paint to fully dry for at least a day.
Sealing. If I'm using polyurethane resin I would simply use a polyurethane clear coat spray. Matte or glossy. Don't over do it with the spray. I would do like 1 light coat every hour. 3 coats total. Hope this helps, if you want you can check out some of my results on my page. I made some prop hands out of polyurethane resin and painted them using this techniques