Finely finished my DIY tarp I made from some very thin material, (most likely polyester). I couldn't track down any mineral spirits, as it is quite hard to find here in Tokyo... Instead I used a 3:1 mix of paint thinner (900ml) and silicone(300ml). The paint thinner is really stinky stuff and I was worried that the tarp would have a strong lingering smell. I'm happy to say that is does not after one full day of hanging in nasty humid Tokyo weather.
The mix set up quite fast, I didn't have as much time as I would have liked to smooth things out with a sponge.
For application, I just mixed the goo in a thick plastic bag then stuffed the whole thing in, kneading like bread to make sure I got full coverage.
I squeezed out as much of the goo as I could, then hung it tight and quickly went to work wiping it with a sponge.
I haven't weight it yet, but it is definitely heavier than before.
I'll try it out soon in the rain, but it seems like it is going to work just fine.