I was wondering if theres any way I can make my Arduino waterproof. I don't want to use a lunch box or something similar because I'd like to keep my project as slim as possible.
I've read about something called "potting compound". Will this keep my Arduino dry in lets say a washing machine? 
What about silicone? Would this work aswel?
Thanks for your answers!
I would be afraid washing machine will damage Arduino mechanically even if you make it waterproof.
Potting compound can affect heat dissipation, so watch out for that, but yes it is commonly used to
protect circuitry that's going to be used outside or in humid environment. Not a reversible process.
There are two sorts of silicone resin, acid cure and non-acid cure. Acid cure will attack/etch any metal
on the PCB, clearly avoid that.
You can use this stuff. Gives an IP rating of IP68. You can also remove it at a later date
ThisIsMeRS:
I was wondering if theres any way I can make my Arduino waterproof. I don't want to use a lunch box or something similar because I'd like to keep my project as slim as possible.I've read about something called "potting compound". Will this keep my Arduino dry in lets say a washing machine?
What about silicone? Would this work aswel?
Thanks for your answers!
In addition to the above answers, you need to have an enclosure of some type to contain the potting compound while it cures. You probably need to consider any external connections to the Arduino. Power? Data in and out? Wires will wick moisture into the Arduino board.
Way to avoid wires is to use a waterproof connector with the inside part of the connector also covered with the potting compound.
Potting compound is very thin so it can get to all parts of your project. Air bubbles are a problem, so you need to take care to pop any before the compound cures.
Most potting compounds have a limited lifetime, even in their original sealed, unused container. Many also have a minimum purchase quantity and in the US, will require a hazardous material charge for shipment.
Paul
Maybe silicone conformal coating?
Not sure how durable it is. There should be lots of videos about waterproofing electronics with it.
Ok...
Heres a simplistic answer...
No heat involved ( ie ...outdoor use in the shade)......dip the board in wax.
Or go military grade ....dip the board in epoxy....
remember to cover the pinouts...for future use...
obviously....not so obviously.. 
You'd have to figure out how to get wires in an out, but check out OtterBox.
Cheers!
Dirk
ThisIsMeRS:
. I don't want to use a lunch box or something similar because I'd like to keep my project as slim as possible.
If there are no sharp edges you can vacuum bag it using potting compound.
Using a hoover and plastic bag minimises the compound used and the thickness.
You can just use water based lube. once dried, it is good for using outdoor environment, but of course, it's not 100% waterproof. I will call it water resistance.