I had quite a journey of mishaps while working on this project. I ruined two Circuit Playground boards and wasted about $150 worth of resin. This resin is expensive, both in money and in time wasted, and mistakes can really hurt! In hopes that you will benefit from my mistakes, here are some things to try and avoid.
Don't Embed your Circuit Playground
I had success a few years back embedding a Trinket inside a resin necklace, so I thought I'd give a try to embedding a Circuit Playground and induction coil. Long story short: this was a huge failure. The resin seeped into the battery port and USB port, and into all the delicate nooks and crannies in the board. After two resin pours and 96 hours of cure time, I was left with a useless hunk of plastic that had eaten my careful electronics build. No bueno.
Be Careful with the Wires
The spot where the delicate fairy light wires go into the resin is very, very vulnerable. I was letting my Circuit Playground dangle while I messed around with the resin, and the light strand broke right off.
This happened to me TWICE with this project. The first time, both strands broke and I gave the project up for lost. The second time, just one strand broke and I was able to dig the second wire out of the resin using a soldering iron (sporting a tip I don't care about). I carefully melted the resin around the wire, then scraped it out and was able to re-solder the broken wire. I reinforced it with a plug of hot glue and am now being extremely careful lest it break again.
Don't Get Sloppy with the Mold
For one attempt, I added a "hockey puck" sized pile of hot glue to the bottom of my mold. The idea was to make a cutout for the Circuit Playground built right into the resin, so I wouldn't need to use a base underneath. This really didn't turn out well. The resin is clear enough that all the misshapen lumps are very visible, and the Circuit Playground doesn't quite fit in there anyway despite my best efforts. I would have preferred to just leave it smooth on the bottom.
Sanding the Resin Surface
For this attempt, I shaped the lights into a fairy shape and then embedded the fairy-light fairy in the resin as though she were trapped in amber. I still think this is a neat idea, and may develop it further, but this one didn't really work out.
With this attempt, I used a chop saw to roughly shape the resin and used sandpaper to smooth off the corners and edges and get that rounded amber look.
No matter how much I sanded and polished (I went down to 2000 grit sandpaper) I could not get the resin to look clear again. It does appear to be possible to get your resin to look clear - lots of people on the internet seem to be able to manage it. I couldn't get there without investing another $100 in an electric buffer, buffing pads and polishing compound.
Mix Up All your Color At Once
As I mentioned on the resin pour page, it's really a good idea to colorize ALL your part A epoxy in one go, so all your layers match each other. Counting drops or eyeballing color will never result in an exact color match.
Stay Away from Mica Powders
Mica powders are opaque pigments that can produce absolutely gorgeous results when mixed into resin. However, if you're adding lights to the resin, do not use mica powder in your mix. Not even a little bit! The mica powder blocks the light entirely, and when used with deep pour epoxy it gives a dull, matte look to your project.
Bubbles in the resin are a very common issue. If your resin cures at too low a temperature, the bubbles don't escape as well. Teeny tiny bubbles are usually not a problem.. unless you put lights inside your resin. Then, the light strands seem to point out every single bubble or flaw in your piece.
My pieces have bubbles despite all my best efforts. I'm living with it. If you're a perfectionist, the way to fix this is to get yourself a vacuum chamber and let your resin cure inside it. 2-5 gallon vacuum chambers are commercially available (I don't own one, yet. So I don't have a specific recommendation). They'll suck all the bubbles right out of the resin, while giving you a dust-free and temperature controlled environment. If you want to do a lot of resin work, this seems like a very good investment.